Stan,

You might try a non-MBZ plastic cap like from PepBoys or Kragen.

The PO of my car had to replace the cap to pass the smog check. The cap held pressure but was so tight, I could barely get it off, partly because I have a little tennis elbow. I replaced it with the nicer metal MBZ cap. It holds pressure but I sometimes see an oily film on the edge of the cap so the seal is not perfect, but it's easier on my elbow.

glenmore
1991 300CE