Too small of air filter on a turbo can create a vacuum at the inlet. On a turbo this is bad this vacuum will actually pull the oil past the seals.
Had a brand new turbo pull past the seals and completely damaged it. Bought a brand new second turbo and it started again to pull oil past the seal again. Pulled air filter off and no more issues. Put the small one back on issues.
Went with a large cone and good to go.
This was after I kept being told -
Drain too small not draining properly
My bike is pushing too much blow by past the rings (check rings)
Leak down and compression just fine
Use a check valve it's leaking back into the turbo. Check valve already installed
Use a smaller restrictor on the turbo
Even put a mechanical scavenge pump on. Same thing
Small filters should not come with kits at all. Even a small journal bearing was affected.
So when the second brand new turbo started to 'leak' I took it to a very large garret retailer who actually sells RCC their turbos. They said let me look at the filter. When I showed them they said that's the problem and explained the vacuum.
I did my own research and sure poop too small of air filter on turbo applications can definitely be a cause.
Sucks it cost a $1300 turbo because of filter too small for the application.
What really pissed me off was being told it's something with what I had done, it's just something with my bike.
Haven't had a problem since moving to a larger filter even after multiple hard ass dyno runs at over 400hp.
Switch now is my .02 on a lot of money spent learning