Per the last two replies, I tried it all! That darn filter cover must have been put on with an air wrench! I finally decided to go ahead and change the oil, leave the old filter intact, then I will try and locate a cap socket today. One of the endearing traits of this MB is that the filter is topside, so I don't lose much oil when I take it off to replace it.

What size cap socket is needed?

Thanks guys!