I've been using normal compressor oil the same stuff that I use in my cheap Chinese shop compressor. It's got an ISO rating of 68. I expect that the electric motor will cark it before the big end. So far I've topped my bottle off about 4 times and it has about an hour and a half of run time on it. The oil is still clear but I will probably change it soon to get rid of any metal filings or chunks. Yes, synthetic is better but good old mineral oil is adequate for a crude loose tolerance machine like this. I've got no idea whether synthetic oil is any less explosive under pressure than the normal stuff but I suspect it is much the same. By the way, the Yong Heng has exactly the same crankcase as my shop compressor. The shop compressor probably has the same conrod and crankpin setup as well. It does, however, have a larger motor bolted to it. I agree with Centercut about using hydraulic oil, it is for hydraulics, lubrication is a secondary function.