Not knowing this engine, I was hoping that your oil cooler temp (typically just behind a water pump) would be closer to the 176 F temp that you had on the bottom rad hose, that would make sense.
Again, I don't know this engine or exactly where you pointed at the oil cooler or that coolant circuit.[for the 220 F temp]

My worry is that IF the t-stats aren't closing off the "by-pass" port/circuit/coolant passage, then the high reading, at the oil cooler, could indicate that some of the hot 220-230 F coolant, in the upper cylinder head area, is getting back into the water pump and NOT getting pushed thru the top rad. hose and radiator= never gets cooled; just gets circulated thru the engine again, and gets "super-heated" some more. (Basically, 100% of coolant isn't getting forced into the top radiator hose)


note: the by-pass coolant circuit, is how an engine gets warmed up(no coolant flow to radiator), from say 40F to 190F, then the t-stats begin to open; BUT it should be totally blocked off by around 205F, by the t-stats, thus forcing 100% coolant to radiator top hose.



Did you, yourself, change t-stats, if so, do you remember any type of lip seal on the sides of the t-stats or do you remember a disc or solid washer attached to the new t-stats?