Today I was doiing some work at my car, and I noticed a bit oil in the expansion tank.

Its like 85 % water coolant and 15% oil... Difficult to explain, but if you put your finger in it deep, then its a bit black , but not that much.

If you open the oil cover lid at the engine, the oil still looks nice black... no traces of water in it...

Is this a normal fenomen, or should I think about a new cylinder head and/or gasket?