OK, now I'm officially concerned. I've owned this car for 5 years and 50,000 miles, and never touched that cap. (Didn't even know it was there for most of that time.) I'd hate to have it suddenly expire over some little maintenance item like oil, especially since there's NOTHING in the owner's manual about it.

For what it's worth, it was picked up at the German assembly plant, then shipped over to the states. The numbers are:

Chassis: 115 117 12 124603
Motor: 616 916 12 030703

Thanks for any further replies!