Hi Warren, As they say, oils ain't oils. I assume we are talking reciprocating compressors here. You do not want detergents in your compressor sump so generally most engine oils are not suitable. No combustion going on so you don't need or want detergents removing things. Synthetic oils  are "cleaner" having low levels of sulphur and carbon so you don't get these precipitating on the valve gear of the compressor. Bear in mind you will also get some carry over into the compressed air so you can end up with precipitation of things like sulphur in your regulators when the pressure is dropped.  Low foaming is highly desirable also.  Compressors tend to operate at high temperatures when used extensively  - they are air cooled so have limited ability to stay cool in hot weather and Charles' law says they are generating lots of heat internally. Hence the higher the top end weight number the better to maintain lubrication.  A low ish weight for your multigrade is nice when the machine is cold.  If you have to use engine oil then the 5W50 synthetic is probably OK for home use but if the compressor is doing a lot of work I'd go for a purpose formulated compressor oil.  Hope this helps, otherwise perhaps two Bex and a longer lie down whilst you think about it.  Cheers.