I used to own an 84 190E that did the same thing. I never did determine the cause of it but on some cases it was bad enough that it soaked the air cleaner. I believe attached to your air cleaner is a type of pressure relief which may allow the oil in.
A tech would know best but its could be a possible cause.
I would be curious what caused it to if it wasnt that relief opening.
02 C32 AMG
84 190E 2.3 5 Spd