Bob,
As Wrenchbender alluded to, there are almost no scenarios that would introduce oil into the coolant other and a leaking transmission cooler, and yours isn't a automatic.
The most likely cause would be a greenhorn adding oil to the engine, and mistakenly dumping some into the radiator!
For any internal oil exchange to take place, oil pressure would have to be greater than coolant pressure. When the rig was shut off, oil pressure drops to zero while cooling system pressure remains. This would result in coolant being forced into the engine, and the resulting milkshake would be a dead giveaway.
Taking your radiator to the shop when it doesn't leak coolant won't prove anything. The block has nooks and crannies that will hold junk, so you will probably have to flush your system a few more times.
As I recall, Bars Leak looks nothing like motor oil in the radiator. Its initial color is white..like soluble oil, but then that dissipates as the pellets are dissolved.
Stuart
