Again, to verify, I've attached pictures.
The first picture shows how I've been sealing. This is the way I bought the car. I simply put things back the way I find them.
The second picture shows how you say I should be sealing the cap.
I hope the fix is this easy. I checked a parts diagram (
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=124066&M=104%2E992&GA=722%2E369503&GM=717%2E450&CT=M&cat=520&SID=18&SGR=045&SGN=02), which verifies there's only one gasket. I wonder where the square seal came from?