250 is a good, safe stopping point for oil temp. Up to 300 F is OK for short durations. The EGT to Oil temp relationship is much more difficult to characterize and depends on the engine, setup, etc. Turbocharged engines will heat the oil up much quicker than NA due to the amount of oil that is pumped through the turbo and used as a coolant for the turbo.
Past experience doing durability engine testing for a small engine OEM. On these particular engines, the failure point due to high oil temps was the piston pin to con rod connection while using 20w-50 oil.
At 250 degs, the engine was good and would run for it's expected life span.
At 300 degrees, the life span of the engine would be shortened slightly, there would be noticeable signs of heat build up on the small end of the rod, but as long as the oil was changed frequently, typically no dramatic engine failures.
At 350 degrees, the engine lifespan was measured in minutes....
