Anyone know of the most common failure leading to coolant in the oil?

 

Head gasket or oil cooler, if you notice smoke in the exhaust, head gasket. If your oil cooler is leaking, that's fairly simple to replace..

 

The o rings in the front timing cover can degrade as well and cause this.

 

Well, at 240k miles something like this can happen. The oil cap has the chocolate milk on the inside. I see the 2 likely problems (other than head gasket) in the thread here, but I have a question: Is there a test to isolate which of those two issues I could have?

 

Oil cooler or radiator.