Originally Posted By: SecondMonkey
Hmm....how does the oil get to the valves? I don't believe they are oil bathed like they are in an engine - I think they're just check valves on top of the cylinder.
Every time the compressor piston travels down, a thin film of oil gets left on the cylinder walls. Next compression stroke, some small part of that oil gets scraped off the walls and pushed through the valves, along with being heated up greatly as the air is compressed. Don't you notice all the oil mixed with the water when you drain the condensate out of your pressure tank?
You really don't want to run motor oil in a compressor, especially not a multi-stage type that heats the air a lot more than a single-stage. The air doesn't get hot enough to burn the oil, but it does tend to cook it and the additives in engine oil can gunk up the valves and collector plumbing. The other thing is that engine oil will try to entrain the water and keep it in solution, but compressor oil will let the water drop to the bottom of the sump and stay there, safely out of the way, until you change the oil.
Quote:
The thing is, I want to use synthetic
Its really overkill for a cheap compressor IMO. But if you insist, Royal Purple's very first product was a compressor oil and they still sell "Synfilm" compressor oil or whatever they call it.