Various engines exhibit the goop on the bottom of the oil filler cap simply due to humidity condensation, i.e. it's not a headgasket leak.

There's about zero humidity where I live so not the best example, but can report never saw this on two different previously-owned M103 engines (what this thread is about) and did see it on an M112 V6.

Wouldn't hurt to peek into the rad overflow to see condition of the antifreeze. It shouldn't be green, BTW.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles