I have a 1987 260E with a leaking head gasket, it has leaked for 50000+ miles and that is with lots of redline driving. The car has been in my family over 4 years like that. No oil in coolant or visa versa, just a prepetually wet side of the engine, towards the rear. Maybe your engine is the same situation. I read that keeping your coolant changed at preoper intervals will prolong the head gasket life, as old coolant can eat the head gaskets due to PH changes or something like that