[jeff5may]

I like mineral oil for a whole lot of reasons.

1. It is in the scavenged R22 compressors and heat exchangers I use in my monsters. No compatibility issues there.

2. It isn't a lover of moisture. The ester and glycol and AB oils absorb moisture and don't let go of it. Once your synthetic nu-oil system sees moisture of any sort, it's time for an oil change (no fun). With my mineral oil system, if something bad happens (leaks suck), I can add back any lost oil (if necessary), pull a deep vacuum for a while, and nearly all the water will be drawn out of the oil. After a double or triple purge/evacuation run, I'm good to recharge with assurance the oil in the system isn't tainted. At least not with moisture.

3. It is pretty well inert. If you have a burnout event, it doesn't break down into gloop, moisture and sludge that clog up everything. POE oil also shares this property, unless there is moisture present anywhere. If i was lubricating a jet engine, I would use POE because that's what it was made for. Of course, at jet engine temperatures, all the water boils right out.

4. It is compatible with pretty much all commonly used (and most uncommon) refrigerants. The other oils may react with whatever is (or was) in your system unless it is spotless on the inside. If I was producing virgin units from brand new parts, this wouldn't be a problem. A quickie blow-out with system flush or brake parts cleaner (non-chlorine) and inert gas, and I'm ready for that new (to me) compressor to braze in. You never know what might have been run through those used units in the past.

5. The viscosity index issue with R22 in cold weather is offset by the solubility of propane. When it gets really cold outside, R22 doesn't thin the mineral oil like R290, so you have that "cold cranking" issue like with your car. With propane, it dissolves in the oil like CO2 in soda pop, so when you start up cold, the oil is thin with dissolved liquid propane. The otherwise normally thick oil flows like water and foams up like soda pop, coating all the parts that lie in its path with a layer of oil as the propane boils out. Once the stuff gets to a pressure where the propane has very much superheat, it separates from the oil, causing the oil to thicken to its "normal" value.

In large, industrial/commercial type systems, some of these reasons don't hold up. Such as in the paper BBP cited, they use hermetic piston or screw compressors instead of rotary or scroll compressors. Most of the oil doesn't circulate through the whole system like in smaller systems like I work with. They have things like oil separators and dedicated oil pumps, which don't work correctly when the oil foams up. In such systems, the other types of oil work better to their reduced miscibility with refrigerant gases.