Higher than around 210-220F for sustained periods will cook the oil...

Even my small block chevy with dino oil, but alum heads, 9.5:1 compression, pump gas, headers, 4barrel, etc., etc., 180F thermostat, L88 radiator, max oil temp was 210F. I think your temp will go down as the engine breaks in...but 260F is another reason to change the oil at 500miles or so.

The army used to measure blowby during run in on tank engines at Letterkenney and Anniston army depot AVDS1790 rebuild...we used to run the engines 5-6 hours at varying power levels...found there was a correlation between blowby reduction and run in time such that if blowby was dropping in the first 20minutes, the engine would continue to break in/blowby would drop to a minimum in about 30-60 hours of operation and then sloooowly increase (or worse if contaminated by dust, etc). Bottom line...we didn't run them in 5-6 hours...just 20minutes on the dyno and shipped back to the field. NO DIFFERENCE in field returns. I don't remember the oil temps over break-in because the engine was oil cooled (four massive oil coolers, about 2feet by 3feet mounted outside the engine...cubed the power pack...air went from the outside of the coolers across the coolers across the air cooled diesel jugs and up the top of engine V-12 engine.

But you can figure about 5-10 hours of break in to minimize blowby, then change to synthetic to maintain seal.