EDIT: NEW INFO this post maybe useless
it loosing coolant/or oil? or it was just the stuff you found made one suspect HG,
so far I think its just old coolant deposits you found or it's had some coolant leak and a leak stopper dropped in the rad like the old Barr turd
I think run it for a bit with clean coolant and see if you get a colour change or more signs of oil
could test the engine oil cooler on the car with one coolant line clamped or blocked and apply some pressure to other line
the oil side of the cooler can take over 100 psi, you only need 15 or 30 psi and see if it drops,
rig up a car tyre shraeder valve to the open coolant line (so it holds pressure) and a mountain bike track pump with gauge preferably