I have a 2003 F150 4.2L with 200,000 miles and am a shade tree mechanic. The engine was replaced with overhauled one about 10,000 miles ago (failed rear main seal and engine damage due to loss of oil pressure), along with all new hoses, new water pump, and a new radiator (and with coolant https://www.rockauto.com/en/moreinfo.php?pk=13527805 if it makes a difference). Recently I was doing a once-over of the engine compartment and noticed there is oil in the overflow bottle. No loss of coolant, no coolant in oil, oil is down just a little from last fill.

a) could this be a failure in the new radiator in the oil coolant section? Any way to test this? What's the likelihood that the new radiator could fail?
b) other option is the overhaul has issues, which I really don't want to have to deal with pulling the engine again. There's warranty coverage but won't cover the full cost if I need to pull it.
c) any way to test if this is a head gasket? I know there are exhaust gas testers - do they work with overflow bottles or just with radiators with pressure caps?

I was going to run some TSP (trisodium phosphate) through the system to clean out all the oil, and then was considering replacing the radiator since that's relatively inexpensive. Any thoughts of what to do would be greatly appreciated.

 

The radiator doesn't have oil coolant section. It has a transmission fluid coil in it..... It actually both heats and cools transmission fluid.

Check the transmission fluid because coolant in the transmission fluid will lead to very bad things very fast.

If you do have oil and not transmission fluid in your coolant then you probably got a head gasket issue