Foaming compressor oil in older 375
Ok, so my boss brought home this older 375 compressor. While I'm servicing it I change the compressor fluid filter and fill with just a 30wt motor oil. Which is what I put in our shop compressors. Start it, let it run for a while. I can't see any fluid in the sight glass so I shut it off, drain it of pressure, and open the compressor fill lid so I can add fluid to bring it to the proper level. Once opened foam slowly started to crawl up and out of the full tube.
1. Should I have not opened the full lid after running it??
2. When do I check the fluid level? When running, or cold?
3.Is there any oil I can put in it that won't foam like that lol? I've read to use atf but idk jack about these large compressors