Vacuum Pump Oil Is Doing More Than Lubricating the Pump

In an oil-sealed rotary vane vacuum pump, oil is not simply there to reduce friction.

It lubricates moving components, carries heat away from the pumping mechanism, helps seal clearances between internal parts, and contributes directly to the pressure the pump can achieve. Edwards describes oil in an oil-sealed rotary vane pump as both a sealant and coolant, while Leybold notes that oil-sealed pumps rely on oil to achieve higher compression ratios and better internal cooling. (Edwards Vacuum)

That makes the oil specification part of the vacuum system.

Put the wrong lubricant into a gearbox and you may shorten bearing life.

Put the wrong lubricant into an oil-sealed vacuum pump and you can also change pump-down performance, ultimate pressure, starting behavior, sealing, oil consumption, and service life.

One of the most important specifications is viscosity.

But viscosity is only the beginning.

What Does cSt Mean on Vacuum Pump Oil?

Vacuum pump oil data sheets commonly give kinematic viscosity in centistokes, abbreviated cSt.

You may also see the same measurement written as:

mm²/s

One centistoke is equivalent to one square millimeter per second, so an oil listed as 68 cSt at 40°C is also 68 mm²/s at 40°C.

The temperature is critical.

Oil becomes thinner as temperature increases, which is why technical data commonly specifies viscosity at both 40°C and 100°C.

For example, Busch publishes its VM 068 mineral vacuum pump oil at 68 mm²/s at 40°C but only 8.5 mm²/s at 100°C. Its VM 100 is approximately 100 mm²/s at 40°C and 10.7 mm²/s at 100°C. (Busch Vacuum Solutions)

The oil did not change products.

The temperature changed.

And that changed the viscosity dramatically.

This is one reason operating temperature matters when selecting a vacuum pump lubricant.

What Does ISO VG Mean?

ISO VG stands for ISO Viscosity Grade.

The number identifies the oil’s nominal kinematic viscosity range at 40°C.

So an oil designated:

ISO VG 32 is centered around approximately 32 cSt at 40°C.

ISO VG 68 is centered around approximately 68 cSt at 40°C.

ISO VG 100 is centered around approximately 100 cSt at 40°C.

Those numbers are viscosity classes—not product names.

Busch, for example, currently sells mineral oils in ISO VG 32, 68, 100 and 150 classes. (Busch Vacuum Solutions)

Vacuum Pump Supply’s own mineral-oil line follows the same general convention. Current published specifications include:

VPS Oil	ISO Grade	Published Viscosity at 40°C
VPL M32	ISO VG 32	33 cSt
VPL M68	ISO VG 68	67 cSt
VPL M100	ISO VG 100	103 cSt

These are real measured or supplier-published viscosity values, which illustrates an important point: ISO VG 68 does not mean every ISO VG 68 oil will measure exactly 68.000 cSt. It identifies a viscosity class. (Vacuum Pump Supply)

Why Does Vacuum Pump Oil Viscosity Matter?

Inside an oil-sealed rotary vane pump there are very small clearances between moving and stationary components.

Oil has to move through the lubrication system while also maintaining an effective lubricating and sealing film.

If the lubricant is significantly different from what the pump was designed to use, those functions can change.

Edwards specifically warns against using low-viscosity oils that are not appropriate for its rotary vane pumps, noting that incorrect low-viscosity oil can negatively affect pumping speed, ultimate vacuum, and pump life. (Edwards)

That is important because the symptom may not immediately look like an oil problem.

A technician may see:

longer pump-down time,

poor ultimate vacuum,

higher oil consumption,

abnormal temperature,

or shortened component life,

and start looking for worn vanes, seals, or leaks.

Sometimes the first question should be much simpler:

What oil is actually in the pump?

What Happens if Vacuum Pump Oil Is Too Thin?

An oil that is too thin for the pump and operating conditions may not maintain the sealing film the pump was designed around.

In an oil-sealed vacuum pump, that can matter both mechanically and pneumatically.

Reduced oil-film effectiveness can contribute to poorer internal sealing, which can affect attainable pressure and pumping performance.

It can also reduce the lubricant’s ability to maintain the intended film between loaded surfaces.

This does not mean that “thicker is always better.”

It means the pump was engineered around a particular lubricant behavior.

Edwards’ rotary-vane service guidance is a useful real-world example: the company specifically connects inappropriate low-viscosity oil with deterioration in pump speed, ultimate vacuum, and lifetime. (Edwards)

What Happens if Vacuum Pump Oil Is Too Thick?

Going the other direction creates a different set of problems.

A much heavier oil resists flow more strongly, particularly when the pump is cold.

That can increase starting load, slow oil circulation, and change lubrication behavior until operating temperature is reached.

This is one reason manufacturers publish both the viscosity grade and temperature-related properties rather than simply labeling a product “vacuum oil.”

Modern synthetic lubricants may be selected specifically because their viscosity changes less dramatically across a wide temperature range. Busch, for example, describes its PAO synthetic VSI 100 as having high viscosity-temperature stability for lubrication at elevated operating temperatures; it publishes the oil at 100.8 mm²/s at 40°C and 14.0 mm²/s at 100°C. (Busch Vacuum Solutions)

This also explains why two oils that both say ISO VG 100 may behave differently as the operating temperature changes.

What Is Viscosity Index?

You may occasionally see another number called VI, or viscosity index.

Viscosity index describes how strongly an oil’s viscosity changes with temperature.

A lubricant with better viscosity-temperature stability maintains its viscosity characteristics over a wider temperature range.

This becomes important in equipment exposed to cold starts, high operating temperatures, seasonal ambient changes, or continuous heavy-duty operation.

For example, Busch publishes a viscosity index of 141 for its PAO synthetic VSI 100. Its mineral VM 022 is published with a VI of 90. These are different oils designed for different applications, but the comparison illustrates why viscosity grade alone does not tell the complete temperature-performance story. (Busch Vacuum Solutions)

Viscosity Is Not the Same as Vapor Pressure

This distinction is particularly important in vacuum service.

An oil can have the correct viscosity and still be unsuitable for the vacuum level you are trying to achieve.

Why?

Because the oil itself has a vapor pressure.

Every liquid has some tendency to evaporate.

Inside a vacuum pump operating at very low pressure, an oil with excessive vapor pressure can become part of the gas load and ultimately limit the pressure the system can achieve.

This is why vacuum oils are formulated differently from ordinary machine lubricants.

Busch specifically identifies low vapor pressure as an important characteristic of its VMA 055/A119 vacuum oil because lower vapor pressure allows a pump to reach lower ultimate pressures. Leybold similarly describes its vacuum oils as formulated for low vapor pressure in addition to thermal stability and oxidation resistance. (Busch Vacuum Solutions)

So:

Viscosity tells you how the oil flows.

Vapor pressure tells you how readily the oil itself contributes vapor under vacuum.

Those are different properties.

Both can matter.

Can I Use Hydraulic Oil or Compressor Oil With the Same ISO VG Number?

This is where ISO grade can become dangerous if it is treated as the entire specification.

An ISO VG 68 hydraulic oil and an ISO VG 68 vacuum pump oil may have similar viscosity at 40°C.

That does not make them equivalent fluids.

A vacuum pump lubricant may be selected or formulated around vapor pressure, volatility, oxidation stability, additive chemistry, gas separation, water handling, elastomer compatibility, and process-gas exposure in addition to viscosity.

Leybold describes its vacuum oils as specifically developed around characteristics including thermal stability, low vapor pressure, oxidation resistance, and application-specific chemistry. (Leybold Global Webshop)

Vacuum Pump Supply’s own SDS and oil-reference resource therefore states explicitly that oil viscosity alone does not determine compatibility. Pump model, lubricant type, process conditions, operating temperature, and chemical compatibility all need to be considered.

That is the correct way to evaluate a substitute.

Mineral vs. Synthetic Vacuum Pump Oil

Two oils can have nearly identical viscosity values but completely different base chemistry.

Consider an ISO VG 100 mineral oil and an ISO VG 100 PAO synthetic oil.

Their room-temperature viscosity may be quite close.

Their performance under high temperature, oxidation exposure, water contamination, or long service intervals may not be.

VPL M100 is currently specified as a hydrocracked and isodewaxed mineral vacuum pump oil with a published viscosity of 103 cSt at 40°C. VPL S100 is a PAO synthetic oil with a published viscosity of 100 cSt at 40°C. (Vacuum Pump Supply)

From a viscosity-number standpoint, they look extremely similar.

From a chemistry standpoint, they are different lubricants.

That is exactly why choosing oil by “100 weight” or “close enough viscosity” alone is not a reliable method.

What About PFPE Vacuum Pump Oil?

PFPE is an even clearer example.

Perfluoropolyether fluids are used where ordinary hydrocarbon oils may be inappropriate, including certain chemically aggressive or oxidizing applications.

Busch describes PFPE fluids as chemically inert and suitable for specific corrosive or elevated-oxygen applications. Leybold similarly specifies PFPE-based LVO 400 for particular reactive and oxidizing service. (Busch Vacuum Solutions)

A PFPE oil may happen to have a viscosity numerically similar to a mineral oil.

That does not make those oils interchangeable.

Changing a pump between hydrocarbon and PFPE service can require manufacturer-specific preparation, cleaning, compatible seals, and application review.

For oxygen-rich, oxidizing, reactive, or chemically aggressive processes, lubricant selection becomes a process-safety decision, not simply a maintenance choice. Use only a fluid and conversion procedure approved for the exact pump and application.

What Does Oil Vapor Pressure Have to Do With Ultimate Vacuum?

Imagine a perfectly tight pump with fresh seals and no process contamination.

As pressure falls, eventually the gases released by surfaces, seals, process materials—and potentially the pump oil itself—become increasingly important.

If the lubricant has significant vapor pressure at operating temperature, oil molecules entering the gas phase can establish a practical pressure floor.

This is one reason oils designed for deeper-vacuum service emphasize low volatility and low vapor pressure.

It is also one reason a lubricant intended for a rough-vacuum industrial pump should not automatically be assumed suitable for a two-stage pump being used as a backing pump in a high-vacuum system.

Edwards describes its Ultragrade Performance 19 oil as intended for small and medium two-stage rotary vane pumps where both good lubrication and high ultimate vacuum are required. (Edwards Vacuum)

The pump, operating pressure, and lubricant have to be considered together.

Why Does Viscosity Change When Oil Gets Hot?

Oil molecules move more freely as temperature increases.

The practical result is lower viscosity.

This is why a pump may behave differently immediately after a cold start than it does after reaching normal operating temperature.

It is also why manufacturers specify viscosity at standardized temperatures rather than simply describing an oil as “thin” or “heavy.”

Take Busch VM 100 again:

at 40°C, its published viscosity is approximately 100 mm²/s.

At 100°C, that falls to approximately 10.7 mm²/s. (Busch Vacuum Solutions)

That is nearly an order-of-magnitude change.

Operating temperature therefore affects how the oil flows through the pump and how the lubricating film behaves.

Why Does Old Vacuum Pump Oil Change Performance?

Oil does not remain chemically and physically unchanged forever.

Heat, oxygen, process vapor, water, solvents, particulates, and reactive gases can gradually alter it.

Oxidation can generate heavier degradation products and deposits.

Process contamination can change viscosity.

Water can create cloudy or emulsified oil.

Light solvents can thin the lubricant.

Other contamination can make it thicker or produce sludge.

So even if the pump originally contained exactly the correct oil, the oil may no longer behave like the original product after extended contaminated service.

Leybold recommends more frequent oil changes when aggressive vapors consume the protective properties of specialty oils and notes that water contamination can prevent some oil-filled pumps from reaching their ultimate pressure. (Leybold)

Edwards likewise emphasizes oil condition as a major part of rotary vane pump maintenance. (Edwards)

Does Dark Oil Always Mean the Viscosity Is Wrong?

No.

Color is a useful maintenance indicator, but it does not identify viscosity by itself.

Dark oil can indicate oxidation, overheating, process contamination, suspended material, or chemical degradation.

Milky oil commonly suggests water or other condensable contamination.

An oil can also appear relatively clean while its chemical or physical properties have changed.

If oil condition is questionable, the safest approach is to determine why it changed, correct the process problem, and replace it with the correct fluid.

For severe contamination, a flushing procedure may be appropriate before adding the new operating oil.

Can Two Different Brands of ISO VG 68 Vacuum Oil Be Mixed?

Not automatically.

Even if both fluids are labeled ISO VG 68, they may use different base stocks and additive systems.

Mixing can change viscosity, foaming behavior, demulsibility, oxidation resistance, material compatibility, and vacuum performance.

Vacuum Pump Supply’s current M-series product pages therefore advise against casually mixing oils and recommend draining the existing fluid and following the pump manufacturer’s change or conversion procedure whenever possible. (Vacuum Pump Supply)

That becomes even more important when changing between mineral oil, ester, PAO synthetic, silicone, or PFPE chemistry.

Why Manufacturer Oil Numbers Can Be Confusing

Manufacturers rarely put the complete technical specification into the product name.

A customer may see names such as:

Edwards Ultragrade Performance 19,

Leybold LVO 130,

Busch VM 068,

Welch DuoSeal oil,

or Agilent DS19.

Those names do not tell you enough by themselves to safely substitute one lubricant for another.

You need the underlying specifications.

That is why an oil cross reference should compare more than branding.

Vacuum Pump Supply’s current oil cross-reference resource includes oil chemistry, viscosity information, typical applications, and replacement direction across Edwards, Leybold, Busch, Welch, Agilent/Varian, Becker, Kinney, and other vacuum pump families.

The correct sequence is:

identify the pump → identify the specified oil → verify chemistry and viscosity → consider the process → then evaluate the replacement.

A Practical Example: ISO VG 32 vs. 68 vs. 100

The three VPS mineral-oil grades provide an easy way to see how significant the viscosity differences are.

VPL M32 is approximately 33 cSt at 40°C.

VPL M68 is approximately 67 cSt at 40°C.

VPL M100 is approximately 103 cSt at 40°C. (Vacuum Pump Supply)

M100 is therefore more than three times as viscous as M32 at the published 40°C test point.

These are not trivial differences.

They are different lubricant grades intended for pumps and applications requiring different oil characteristics.

This is why pouring “whatever vacuum oil is on the shelf” into an oil-sealed pump can produce disappointing results even though the bottle says vacuum pump oil.

Frequently Asked Questions

Is ISO VG 68 the same as 68 cSt?

ISO VG 68 identifies a viscosity grade centered around approximately 68 cSt at 40°C. The actual published viscosity of a specific oil can vary within the grade. For example, VPS M68 is published at 67 cSt at 40°C, while Busch VM 068 is published at 68 mm²/s. (Vacuum Pump Supply)

Is cSt the same as mm²/s?

Yes. For kinematic viscosity, one centistoke equals one square millimeter per second.

Is ISO VG 100 thicker than ISO VG 68?

Yes, at the same reference temperature. An ISO VG 100 lubricant has a higher nominal kinematic viscosity than an ISO VG 68 lubricant.

Does thicker vacuum pump oil create a deeper vacuum?

Not necessarily.

Ultimate pressure depends on pump design, internal condition, oil sealing behavior, lubricant vapor pressure, temperature, gas load, contamination, and other factors.

Using a heavier oil than specified is not a reliable way to improve vacuum.

Can I substitute an oil if the cSt value matches?

Not based on cSt alone.

You also need to verify base chemistry, vapor pressure requirements, pump design, additives, operating temperature, process gas compatibility, seal compatibility, and manufacturer requirements. VPS’s current SDS resource specifically warns that viscosity alone does not establish compatibility.

Why does vacuum pump oil need low vapor pressure?

The oil is exposed directly to vacuum. An oil with excessive vapor pressure can itself contribute gas to the system and interfere with the pump’s ability to reach lower pressure. Busch specifically identifies low vapor pressure as an important characteristic of oil intended for lower ultimate-pressure operation. (Busch Vacuum Solutions)

Is synthetic vacuum pump oil always better than mineral oil?

No.

Synthetic oil can provide advantages in specific applications, including thermal stability or wider-temperature performance, but the correct fluid is the one specified or approved for the pump and process.

A mineral oil is not automatically inferior, and a synthetic oil is not automatically compatible.

Can I use automotive oil in a vacuum pump?

Do not select oil for an oil-sealed vacuum pump simply because an automotive lubricant appears to have a similar viscosity. Vacuum pump oils are chosen around vacuum-specific requirements including volatility, vapor pressure, sealing, oxidation behavior, contamination exposure, and pump-material compatibility.

The Bottom Line

The viscosity number on a bottle of vacuum pump oil matters.

But it does not tell the whole story.

ISO VG tells you the viscosity class.

cSt tells you the measured kinematic viscosity at a stated temperature.

Viscosity index tells you something about how viscosity changes as temperature changes.

Vapor pressure affects how the oil behaves at low pressure.

Base chemistry determines how the lubricant interacts with heat, process gases, contamination, seals, and the application.

That is why the correct question is not:

“Do you have a 68-weight vacuum oil?”

The better question is:

“What oil does this exact pump require in this application?”

Vacuum Pump Supply’s oil pages now include viscosity and oil-type specifications for many VPS, Edwards, Leybold, and specialty fluids, along with an expanding OEM oil cross-reference and SDS library. (Vacuum Pump Supply)

For oil selection, send the pump manufacturer, complete model number, current oil name or part number, and process information if the pump handles unusual gases, solvents, moisture, oxygen-rich service, or aggressive chemistry.

That allows the oil to be matched to the pump instead of matching a bottle by number alone.

Useful internal links for this article are the Vacuum Pump Oil & Fluids category, the Vacuum Pump Oil Cross Reference, and the Vacuum Pump Oil Safety Data Sheets library. Product-level links can also go directly to M32, M68, M100, and the relevant OEM oil when the article discusses a real viscosity example.