I have the Mercury 500 EFI's in my boat and recently have had problems with one of the engines losing oil from the dip stick and valve cover vents. I was running pretty hard when I suddenly lost oil pressure. I shut the engine down and lifted the hatch to find my port engine lost most all of its oil suddenly. I could not find a broken oil line or anything obvious so I put new oil in the engine and started it up. The engine holds oil fine but when the engine exceeds approx 3k rpm the engine begins losing oil again. I checked the PCV valve and it seems to be fine. There also doesn't appear to be any water in the oil either. Any ideas on what may be the problem?

sounds like burnt piston,time for a leakdown test.

sounds like you are getting alot of pressure in crankcase, I second the motion, do a cylinder leak down test. Make sure you don't have any plugged pcv system.

I had same problem with one of my engines last summer. Turned out to be a cracked piston which in turn hurt the cylinder wall. Good luck