Today's oils there are no worries in terms of "cooking" your oils. So much of these worries is a carry over from the old engine days where 180 even 160 degree Tstats were the norm along with finding that sludge in your engines. Back when Carbs and points were the norm not fuel injection and electronic ignition.
Oil temps in the pan should be ideally 212-230 degrees. At peak it can go higher and of course until you warm up, lower. The point is ideal oil temps are in this range. I did a quick check to see if I could pull up some of my old Diesel data on oil temps but I must have them on a different PC. It showed engine wear at the various temps etc and what range offered the best wear plus efficiency etc.. I did a quick check and found some data from Cummins. Its actually on big rig fuel economy but in one of the graphs they show what is suppose to be "normal" oil temp. That temp is 230F. Also you will note that this is temp in the oil pan not the in the engine block say before the oil cooler where it will be even higher for a short period. Point being this is considered normal.
Here is the link and go down to page 13 where the graph is labeled "Effects of Lubricant Temperature and Performance"
https://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf What they are going into there is how because syn have higher flow ability at low temps they help fuel econ but forget all that as its not what were are looking for here. Look at the oil temp range and what is considered normal and thus what cummins sets there engines to reach and maintain for oil temps.
So if you look at a 203 and a 195 first off were are only talking about 8 degrees here its not some huge variation like going from say a 203 to a 180. 203 would be fully open around 215-220. For cruising and low load its very possible and likely the Tstat will never fully open as it stay cool enough. But we are looking at peak effects. So 220. Figure oil temp of 10-15 over that so 230-235. So that is your under reasonably heavier loads.
Further if we take a look at stock tuning calibrations timing is not cut back until oil temps reach 260 degrees and that is again at the hpop. So I think it would be safe to say that is too hot at least in terms of the HEUI system and conventional oils.
There are plenty of trucks even those with hybrid injectors turbo custom tunes that run lower oil temps than coolant temps at all times other than under heavy loads ( towing etc) or hard runs. Something to consider when you are thinking about how well your oil cooler is working also consider what your EGTS are looking like as that can also give you an idea of how much more heat you are seeing.
I saw data from a guy that had some typical performance upgrades custom tunes etc running a CGW of 18000. He basically floored up a huge very long steep grade up to a peak elevation of 11K It was something like a 10-12 min run straight. EGTS were running constant at over 1300 F His oil temp peaked @ 253. Same rig going over Vail pass slightly lower and and less grade only hit 230F same load. Cruising on more normal grades same load was around 200-205.
At those temps the T-stat temp would have made no difference as a 203 even a 205 T-stat would have been fully open. But it does give you an idea of the range these engines can see from light cruising to full out maxed running harder that you likely really should push things regularly. Temps can vary wildly depending of conditions. Point is for them to get so hot as their is an issue the T-stat is not going to be in play unless you have a clogged oil cooler.
Also do not forget when you are comparing temps people post to know here the temp is being taken from. IH and Ford take if from the HPOP res. Obviously if you pull from the block before the cooler are you going to see higher temps than just about anywhere else in terms of readings