There are 2 kinds of engine stop leak. One is an oil viscosity improver (OVI) like STP. It basically is a very thick oil -- you can guess how it works.

The other is an additive that swells seals -- similar to tranny stop leak. This is the stuff you should try. It sound to me like your rear main seal may be expiring, which ain't a cheap fix. However, be sure to check around your valve cover gaskets!

__________________
'01 SLK320, '79 450SL & '01 C320 -- What? 3 Mercedes? I am DEFINITELY crazy!!!