Fixed a leak on a MTZ80 that had a crack in the discharge line just after compressor and it lost some of it's oil. I added some (maybe 700cc of 1800cc total capacity) but I think it is still pretty low on the sight glass. There is another unit running side by side and it too seems to be low on oil. What is normal level on these when running and when they're off? I can just barely see the level at the bottom of the glass and when it's running it kind of splashes around that low level. Should I add more? Any good tips on how to add oil without evac the compressor and pump it through the oilplug?