Rather than start another thread with the same problem I mentioned I had this problem with an 88 300E (200K km) I recently bought. Intended as a parts car I went over it and it turned out to be in better condition than my plated 88 300E.

That said I have a brown mess in my reservoir as well. The car does not smoke in the least. Runs smooth, does not overheat, does not boil the rad - no smoke whatsoever from tailpipe. Oil is clean, good oil pressure, no sign of water or coolant after an oil change or on the dipstick. I have to drain and flush rad and lines and start fresh. (assume ten minute flush product is ok?)

Wishful thinking has me hoping somebody put oil in the reservoir by mistake. (former owner fairly car clueless).

I'm ruling out an oil cooler in an 88 300E which spells either transmission fluid making the coolant a milky thick brown stuff or worst case scenario head gasket. Without water in oil or white smoke to the tailpipe how can it be a head gasket would be my last question?