The compressor nameplate and OEM fluid specification always win over any general chart. Intervals below are field starting points for trained technicians — confirm against the manufacturer’s documentation and, on critical machines, against oil analysis. Never mix fluid bases to “get through the week.”
Base oil types
Mineral (Group I/II)
Refined petroleum · 1,000–2,000 hReciprocating compressors, light-duty or intermittent shop units, and machines where OEM specifies a straight non-detergent mineral oil.
Watch out: Oxidizes fastest of the common bases. Above roughly 200 °F sump temperature, varnish and carbon on valve plates build quickly and shorten the interval.
PAO synthetic (polyalphaolefin)
Synthesized hydrocarbon · 4,000–8,000 hRotary screw packages running high duty cycles, hot rooms, and cold-start environments — PAO holds viscosity across a wide temperature span.
Watch out: Poorer seal swell than mineral oil. Old, hardened nitrile seals can shrink and weep after a mineral-to-PAO conversion; plan on shaft seal and o-ring attention.
Diester / polyol ester
Synthetic ester · 6,000–8,000 hRotary screw units where extra cleanliness is wanted — esters are mildly detergent and keep the separator and cooler free of deposits.
Watch out: Hygroscopic: absorbs water, and hydrolysis raises TAN. Aggressive toward some paints, viton grades and epoxy sump coatings. Never leave in a machine that sits for months.
PAG (polyalkylene glycol)
Polyglycol · 8,000–12,000 hLong-life OEM fills on large rotary screws where the manufacturer specifically calls out a polyglycol fluid.
Watch out: Not miscible with hydrocarbon oils. Even a small mineral or PAO residue can gel or drop out sludge. A PAG conversion means a full flush, filter, separator and hose change.
Silicone
Polysiloxane · Per OEMOil-less-adjacent and specialty applications, plus some vacuum pumps where OEM calls for it.
Watch out: Very poor boundary lubrication for steel-on-steel. Never substitute into a standard screw or reciprocating airend.
Food-grade H1 (USP white / PAO H1)
White mineral oil or H1 PAO · 1,000–4,000 hFood, beverage, and pharma plants where incidental air contact with product is possible.
Watch out: Additive packages are lighter than industrial equivalents, so oxidation life and anti-wear performance run shorter. Do not stretch the interval to industrial numbers.
Mixing compatibility chart
| In machine ↓ / adding → | Mineral | PAO | Diester | PAG | Silicone |
|---|---|---|---|---|---|
| Mineral | OK | Caution | Caution | Never | Never |
| PAO | Caution | OK | Caution | Never | Never |
| Diester | Caution | Caution | OK | Never | Never |
| PAG | Never | Never | Never | OK | Never |
| Silicone | Never | Never | Never | Never | OK |
Caution means chemically miscible but not recommended: the mixture inherits the shorter service life and the weaker additive package, so drop back to the shorter interval and change out at the next PM. Never means a full drain, flush, filter and separator change is required before switching.
Picking a viscosity grade
ISO 32
Rotary screw, cold rooms or low discharge temps; some vane units.
Thin film — do not use in a hot reciprocating pump.
ISO 46
The default for most rotary screw packages and many recips.
If the nameplate is unreadable, ISO 46 is the safest generic starting point.
ISO 68
Hot ambients, high-pressure two-stage recips, worn pumps burning oil.
Heavier film costs a little efficiency and slows cold-start flow.
ISO 100+
Large slow-speed reciprocating and some vacuum pumps per OEM.
Only when OEM specifies — too heavy for typical screw airends.
Change intervals
Halve any interval for sump temperatures consistently above about 195 °F, dusty or humid rooms, or duty cycles above 80%. Use the oil aging calculator to estimate remaining life from operating temperature and hours.
Frequently asked questions
Can I mix synthetic and mineral air compressor oil?
Mineral, PAO and ester bases are chemically miscible, so a small top-off will not gel — but the blended fluid takes the shortest life of the two and loses the synthetic's oxidation margin. Treat any mix as mineral-interval fluid and change it at the next opportunity. PAG and silicone must never be mixed with anything else.
What happens if I put PAG in a machine that ran PAO?
Polyglycol and hydrocarbon fluids separate. You get sludge in the sump, a plugged scavenge line, and a separator element that loads within hours. A PAG changeover requires draining hot, a flush charge, new filter and separator, and usually new hoses because residual hydrocarbon in the rubber keeps bleeding back.
Can I use motor oil in an air compressor?
No. Automotive engine oils carry detergents and dispersants that hold water in suspension, plus additives that foam under the aeration of a compressor sump. That drives carryover, varnish and, in reciprocating pumps, carbon on the valve plate — a known cause of discharge fires.
How do I know the oil is done before the hour meter says so?
Send a sample. TAN rising more than about 0.5 mg KOH/g over the new-oil baseline, viscosity moving more than 10% off grade, water over 500 ppm, or visible darkening with a burnt smell all mean the change is due regardless of hours. Oil analysis is cheaper than an airend.
Does a hotter sump really cut oil life in half?
Roughly, yes. The common rule of thumb for oxidation is that every 18 °F (10 °C) above about 160 °F halves fluid life. Fixing a fouled cooler or a stuck thermal bypass valve is usually the highest-return oil-life action on a hot package.