oil cap on my 1992 300ce3 m104 is also a tight little bugger.
I have a large wrench that I use to just give it enough to loosen when I need it.
I have founf some fo the va;ve cover paint underneath is not there. I would assume heat makes it stick. unless mine is leaking I'm not going to bother changign the cap for a new one.
I'll just use my trusty wrench for the added torque to get that sucker off.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.