Can you mix PAG and mineral oil in a rotary screw compressor?
Summary
No, polyalkylene glycol (PAG) and mineral compressor oils are incompatible, so a rotary screw compressor changeover needs a full drain and flush, never a top-up. The rule holds in both directions and for residue from the previous fill; new oil filters, separator and seals belong to the same job.
Chevron's base oil compatibility table rates PAG plus mineral oil as "Never" compatible, and PAG plus PAG only as "Possible" (Chevron, 2016).
In Chevron's mineral-to-PAG procedure, the first PAG charge runs under no load for 8 to 24 hours before it is drained as a flush (Chevron, 2016).
ExxonMobil's flushing procedure runs the flush oil for a minimum of four hours at a discharge temperature of about 85°C before draining (ExxonMobil, 2015).
After a PAG changeover, Chevron monitors make-up fluid, temperature, pressure and filters for two weeks, then samples monthly for six months (Chevron, 2016).
When can PAG and mineral oil meet in a rotary screw compressor?
PAG and mineral oil can meet in a rotary screw compressor only as a small residue left after a drain, flush and filter change, and only when the new oil's supplier accepts that residue. Chevron's primary recommendation for compressors is to drain and flush whatever the current and replacement base oils, partly because a compressor reservoir usually holds a small volume (Chevron, 2016). One PAG and ester compressor fluid data sheet says the fluid "is not compatible with mineral oil based fluids, or other synthetic compressor fluid chemistries and should not be mixed or contaminated with these fluids" (Petro-Canada, 2013).
The table below decides whether a planned oil change in a screw compressor is acceptable.
Condition | Verdict | Why |
|---|---|---|
Top-up of PAG into a mineral fill, or the reverse | No | PAG plus mineral rated "Never" compatible (Chevron, 2016) |
Changeover after a drain only | No | Some compressors do not permit complete draining, so flushing or cleaning is required (Petro-Canada, 2013) |
Drain, flush run, new oil filters and air/oil separator | Yes, if filters and separator come out clean | Documented changeover method (Chevron, 2016; ExxonMobil, 2015) |
Residue left after the flush | Only if the new oil's supplier confirms it by a compatibility test | None of the cited procedures gives a residue limit; test per ASTM D7155-20(2026) |
Seals that saw the previous chemistry | No, until replaced | Chevron says all seals need replacing on a PAG change (Chevron, 2016) |
Paint and flexible sealants | Only if the compressor maker approves them for PAG | Chevron refers paint and sealant compatibility to the compressor maker (Chevron, 2016) |
The key row is the top-up: no oil maker cited here allows PAG and mineral oil to be added to each other in service. Adding either oil to a compressor filled with the other therefore turns routine make-up into a full changeover job.
What happens when PAG and mineral compressor oil mix?
When PAG and mineral compressor oil mix, the two base oils do not form one fluid, and the mixture can gel and drop additives out of the oil. One PAG screw compressor oil data sheet states that the product "is not miscible with mineral oil, PAO, ester based lubricants" (STRUB, 2018). Chevron explains the chemistry behind a PAG-to-mineral change: residual PAG fluids "are highly polar and any residual PAG fluid can dissolve any additives from the fresh mineral hydrocarbon lubricant or synthetic PAO fluid and form a gel" (Chevron, 2016). Schaeffer warns that when petroleum or synthetic fluids are mixed with PAG blends, "the possibility of gelling of the products can occur" (Schaeffer, 2018).
ASTM describes what an incompatible mixture does inside a lubrication system: it "most often forms a precipitate", which forms deposits and can plug filters and oil passageways (ASTM D7155-20(2026), section 5.1.1). That practice covers turbine oils, so for compressor oils it shows the failure pattern rather than a compressor limit.
Seals are the second consequence. Chevron states that because PAG compressor fluids have significantly different chemistries, oil leakage can occur from shrinkage of seals that had swollen in the previous oil (Chevron, 2016).
How do you switch a rotary screw compressor between PAG and mineral oil?
Switching a rotary screw compressor between PAG and mineral oil takes a hot drain, a flush charge, a second drain with new filters and separator, and new seals, following the compressor maker's procedure. Chevron's procedures for both directions, condensed (Chevron, 2016):
Run the compressor to normal operating temperature for 1 hour, then shut it down. Before opening the system, isolate and depressurize it under the maker's lockout procedure and the local rule, for example US OSHA 29 CFR 1910.147, and read the SDS of both oils for hot-oil handling.
Drain the reservoir, oil cooler and filters; disconnect lines at low points to drain trapped oil. Replace the air/oil separator and oil filters, and keep the drained oil for disposal under the SDS and local waste rules.
Fill to half capacity, or the maker's minimum, with the new oil or flush fluid and run 8 to 24 hours while watching temperatures, pressures and filter condition.
Drain again, inspect internal parts for deposits, clean them and replace seals.
Refill to half capacity, run at least 1 hour, drain and change filters and separator; repeat if needed.
Fill to the normal level.
ExxonMobil adds that for a switch from PAG to its PAO-based oils, a diester fluid gives optimal flushing (ExxonMobil, 2015). For when a flush is needed on any oil change, see Do you need to flush the hydraulic system before switching oil?.
What signs should you watch for after a PAG changeover in a screw compressor?
After a PAG changeover in a screw compressor, watch the oil filters, the air/oil separator, seal leaks and make-up oil, because a residue reaction usually shows up there first. ExxonMobil checks the oil filters and air/oil separator for cleanliness and deposits after 4 to 12 hours on the new charge, and repeats the flush if they are not clean; a 10-micron membrane filter pad on an oil sample serves as a visual aid (ExxonMobil, 2015).
This table sorts what you see after the changeover.
Sign | Points to | Action |
|---|---|---|
Deposits on filters or separator | Residue reacting with the new oil | Repeat the flush (ExxonMobil, 2015) |
Rising pressure drop across oil filters | Gel or loosened deposits (Chevron, 2016) | Change filters, sample the oil |
Leaks at shaft seals or gaskets | Seals shrinking after a chemistry change | Replace seals (Chevron, 2016) |
Sample separates into two layers | Base oils not mixing; PAG forms two phases with other oils (ExxonMobil, 2015) | Drain and flush again |
The first row is the main check: deposit-free filters and separator are the pass condition in the ExxonMobil procedure. Chevron then monitors make-up fluid, temperature, pressure and filter condition for two weeks and samples the fluid monthly for six months (Chevron, 2016). Lubinpla's compatibility evaluation judges the residue of the previous product as a separate axis and returns the limits and signs to watch for each conditional combination.
Frequently asked questions
Can you mix two different PAG compressor oils?
Sometimes. Chevron rates PAG plus PAG as "Possible", not assured, because additives differ even on the same base oil (Chevron, 2016). Ask both suppliers for a compatibility test before mixing.
Is PAO compressor oil compatible with mineral oil?
Usually. Chevron rates PAO plus mineral oil as "Possible" and says mineral oils and PAOs are usually compatible, while PAG is not compatible with either (Chevron, 2016).
Which seal materials does a PAG compressor fluid tolerate?
It depends on the product. One PAG and ester screw compressor fluid lists Viton, high nitrile, Buna N, nylon, Delrin, PBT and Teflon as compatible (Gulf Compressor PG data sheet, 2021). Check the data sheet of the fluid you buy.