yeah.. mine does the same.
I nearly died when I bought the thing, but the previous owner rang me to tell me that it wasnt anything to worry about. its actually condensation forming inside the rocker cover.... made worse by having a chrome rocker cover.
chrome dissapates heat in a different way than just plain metal, therefore it gets colder, and condenses more.
I just took the rocker cover off and cleaned it out... lots of grey sludge on the inside of the cover, but none on the motor....
its forming again, but I do longer runs in my mini than the previous owner who was 5min from work, so its only a little bit of grey jelly there now.
clean it out... and dont stress about it, just check if every so often.
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...its leaking oil... and thats a good sign!
It lets me know that I havent run out yet. -Tzi 4-9-04