I just noticed some black/grey spots in my overflow tank, and after taking off my rad cap, some on the bottom of that, I believe it is oil. How can this get into the cooling system? I have no external leaks, which seems to be common with a dowel pin o-ring leaking, and my oil looks great, nice and golden with no signs of water/coolant getting into it. I don't have any issues with cold starting, which this morning was below freezing, and it sat for 3 days since last running. I tried the bubbling in the cooling system test, and I didn't see one bubble in a couple minutes of running with the rad cap off. Is this likely from a source other than the motor?

 

its just the water pump seal/crap. due to the design of the engine its very hard for oil to get into the coolant and vice versa.

 

That is what I was thinking, I know of no seal that seals engine oil from coolant where the oil is under pressure.