BlackMDK

Is it white/yellow stuff on oil cap also normal?

Buying used BMW

I decided to buy a BMW so other day I checked two of them out and I’m bit confused with one issue. I usually always remove the oil cap while engine is running but I noticed that when I removed oil cap engine revs drops (this is 320 E36) on one it dropped so significantly that engine almost staled. Is this normal behavior for BMW.