So in the midst of doing a coolant flush and on the 3rd fill and drain, after filling the stuff that came out into a 1 gallon milk jug for disposal I notice a film of oil on the top of the mostly distilled water the next day. Dip my finger in it and its surely oil. Engine oil or Trans fluid?
Grabbed a rad pressure tester and ran the test twice - holds 16psi for 15 minutes and also a 30 minute test. No leakage.

So, no head gasket leak then apparently. Engine idles/runs well other than...

Oil in Coolant Mystery - 2000 3.8 Mustang