Y'all, I am new to the 'big' engine arena (over 0.1") and have just bench tested a 0.40"

I elected to use the exhaust from the muffler back to the tank and noticed a great deal of black burnt oil being fed into my clean tank of fuel!

It suddenly struck me - why do we do this? Am I doing something wrong? Should I be using some kinda air filter for this application.

I have been testing an OS Max-H 40

TIA

Shobosh