Stevebfl replied to my email and he said:
I suspect engine oil. I suspect that as the clean oil migrates between the head and gasket it is discolored. I have seen this on a 600SEL and never saw the final answer as the car was traded.
I think this is a very small leak. I would dab out the tank and watch it. BTW, oil or tranny fluid getting into the cooling system is better than the other way around. If you had any volume at all there would be a milkshake mess, either chocolate or strawberry (bg).
No laughing matter, but I think you have time to plan. I would not do the valves unless you have over 150k. There is no problem with oil consumption on these motors (that I know of). I would redo all the chain and rails accessible from the top.
Steve
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He confirmed what I later measured; little 'brown' oil droplets in the coolant expansion tank. The size, about 0.03", very small indeed but still there! I could see no evidence of coolant in the oil (chocolate or strawberry milkshakes!). For that, I'm very happy!
Donnie, I will monitor it closely. Doesn't appear to be ATF but thanks for your reply.
This next part may be considered as "
science fiction", so bare with me!
As I was searching the Internet, found a product by K&W called "
BLOCK SEAL". Purported to seal aluminum blocks, radiators, and fix head gaskets! Talk about the perfect snake oil! Well, couldn't pass it up. Got a can for $6.99 at PEP Boys and followed the instructions exactly. Here's the link:
http://www.kandw.com/kwproducts.html
The catch is that this stuff does not work with anti-freeze, only water! Therefore, used some cooling system flush and had the whole system 'power' flushed until the system contained just clear water. Cost $45!!
Then added
Block Seal as directed and drove/idled for about 45mins. Then 'power' flushed it all out again and is currently 'drying' overnight with the system open (radiator cap off, thermostat out and bottom radiator hose off. No coolant or water in the system. Will let it set for 12 hours or so.
Tomorrow night is the last part of this story, good or bad!