AMSOIL guide
Compressor Oil Guide
Air compressor oil is graded on the same ISO viscosity scale as hydraulic oil, but the chemistry is tuned for very different conditions: high discharge temperatures, atmospheric water that condenses into the sump, and long uninterrupted run hours. AMSOIL 100% Synthetic Compressor Oil ISO 100 (PCK) is the standard pick for reciprocating piston compressors, and the same family covers ISO 32, 46, and 68 for rotary screw and rotary vane applications. The synthetic base resists oxidation and varnish, demulsifies condensed water for easy drain-off, and runs up to 8,000 hours in rotary screw service.
Shop AMSOIL Synthetic Compressor Oil
Reciprocating vs rotary screw: two very different lubrication jobs
A reciprocating piston compressor lubricates the crankcase, cylinder walls, valves, and bearings under a duty cycle that swings from cool idle to hot pump-up. The oil sees blow-by and condensed water in the sump. Most reciprocating compressors call for an ISO 100 or heavier compressor oil, often with seasonal allowance for ISO 68 in cold-climate operation.
A rotary screw compressor floods the compression chamber with oil, which seals the rotors, removes heat of compression, and is then separated from the discharged air through a coalescing separator before returning to the sump. The oil runs continuously at elevated temperature with very high air contact. Rotary screw compressors typically call for ISO 32 or ISO 46, and the separator, bearing, and varnish-life targets all push toward a high-quality synthetic.
AMSOIL synthetic compressor oil lineup
| Grade | Typical compressor type | Where to buy |
|---|---|---|
| ISO 32 | Rotary screw, rotary vane, indoor stable temperature | Buy online | Product details |
| ISO 46 | Rotary screw, rotary vane, warmer ambient or heavier duty | Buy online | Product details |
| ISO 68 | Reciprocating in cold climate, rotary in hot ambient | Buy online | Product details |
| ISO 100 (PCK) | Reciprocating piston, most shop air compressors | Buy online | Product details |
| Sirocco ISO 32/46 | Compressed-air dryer and ventilation blower applications | Buy online | Product details |
Synthetic vs conventional compressor oil
Conventional mineral compressor oil is built from Group I or Group II base stocks. It works, but the oxidation life is short at the temperatures a rotary screw or hard-working reciprocating compressor produces. The byproduct of that oxidation is varnish that plates onto valves, the separator element, and the bearings, which translates into shorter separator life and earlier overhauls.
The synthetic base in AMSOIL Compressor Oil resists oxidation by design. The same base shrugs off the high air contact and elevated temperature that turn mineral oil into varnish, so separator change-out intervals stretch and bearing wear stays low. Operators who track oil analysis numbers see oxidation, TAN, and wear-metal trends that justify the extended drain on paper.
Condensed water, demulsibility, and sump drains
An air compressor is essentially a dehumidifier with extra steps. Atmospheric humidity gets pulled into the intake, compressed, and a portion of that water condenses out of the compressed air stream into the oil sump (reciprocating) or into the air receiver and drip legs (rotary screw). The compressor oil's demulsibility additive is what lets that water drop to the bottom of the sump and exit through the drain valve instead of emulsifying into a milky mix.
Two operating habits help. First, drain the sump or the air receiver on a schedule, not just when the milky-oil symptom appears. Second, if the compressor sits unused for weeks, run it to operating temperature periodically to drive moisture out of the oil before it ages the additive package.
How to change compressor oil
Run the compressor to operating temperature, then power down.
Warm oil drains faster and carries more of the suspended water and contaminants out with it. Bleed off all air pressure and disconnect power before opening the sump.
Drain the sump completely.
Position the drain bucket, open the drain valve, and let the oil drain for the full time the OEM manual specifies. Tilting a reciprocating compressor slightly toward the drain side helps the last quart or two come out.
Change the air-oil separator and oil filter if applicable.
On rotary screw compressors, the separator element and oil filter are part of the service. On reciprocating compressors, clean or replace the intake filter and inspect the check valve. Reinstall every plug to the OEM torque spec.
Refill with the ISO grade on the data plate.
Pour the new oil to the sight-glass full mark. Run the compressor unloaded for a few minutes, then top off if the level drops. Log the hours and the date on the unit so the next interval is easy to schedule.
Bottom line: which compressor oil should you use?
- Reciprocating shop air compressor (60 to 80 gallon). Pick AMSOIL Synthetic Compressor Oil ISO 100 (PCK) (also online from AMSOIL).
- Rotary screw compressor, indoor stable temperature. Pick AMSOIL Synthetic Compressor Oil ISO 32.
- Rotary screw or rotary vane, warmer ambient. Pick AMSOIL Synthetic Compressor Oil ISO 46.
- Hot-climate reciprocating or higher-duty rotary. Pick AMSOIL Synthetic Compressor Oil ISO 68.
- Compressed-air dryer or ventilation blower. Pick AMSOIL Sirocco ISO 32/46 Compressor Oil.
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Compressor oil FAQ
Can I use motor oil in an air compressor?
Not as a long-term solution. Motor oil carries detergents that hold combustion soot in suspension, which a compressor does not need and which can foam in the crankcase. Motor oil also lacks the demulsibility tuning a compressor oil has for separating condensed water. Use a dedicated compressor oil, like AMSOIL Synthetic Compressor Oil ISO 100 (PCK), in any compressor that sees regular duty.
Which ISO grade does my compressor take?
The compressor's data plate or owner's manual lists a specific ISO VG, usually ISO 32, 46, 68, or 100. As a general guide, most rotary screw compressors run ISO 32 or ISO 46, most reciprocating piston compressors run ISO 100 or ISO 150, and high-temperature applications step heavier. Always confirm with the data plate before ordering.
How often should I change compressor oil?
Conventional mineral compressor oil typically runs 500 to 2,000 operating hours per change, with shorter intervals for hot or humid environments. AMSOIL Synthetic Compressor Oil is rated for up to 8,000 hours in rotary screw service and up to 2,000 hours in reciprocating service. Pair the extended interval with periodic oil analysis if the duty cycle is heavy.
Why does my compressor oil look milky or have water in it?
Compressing air condenses moisture out of the atmosphere into the crankcase. A compressor oil with good demulsibility lets that water separate to the bottom of the sump where it can be drained off through the drain valve. If the oil emulsifies into a milky mix, the demulsibility additive is depleted and the oil is due for a change.
Will AMSOIL Synthetic Compressor Oil cause varnish?
The opposite. The most common cause of varnish in a compressor sump is oxidation of conventional mineral oil at elevated discharge temperatures. The synthetic base in AMSOIL Compressor Oil resists oxidation by design, which is why rotary screw operators see longer valve, separator, and bearing life on a synthetic.
Is AMSOIL Compressor Oil food-grade?
The PCK family is industrial, not food-grade. For food-processing compressors that require NSF H1 registered lubricants, use a dedicated H1 compressor oil. Browse AMSOIL's compressor oil page for the current lineup and food-grade options.
Where do I buy AMSOIL compressor oil?
Order online from AMSOIL on the compressor oil page, which lists every ISO grade in synthetic format. The matching compressor oil overview on Synthetic Oil Distributor covers the same lineup with side-by-side spec comparisons.
Order AMSOIL Synthetic Compressor Oil
Order online from AMSOIL's Synthetic Compressor Oil ISO 100 product page. To unlock up to 25% off, add an AMSOIL Preferred Customer membership to the same cart and qualifying products switch to member pricing immediately, with no separate sign-up step.