Two things I've heard from respected sources on oil temps, petroleum based oils will start to change molecure structure, go bad, close to 300F and be destroying itself at 325F.
Molil 1 synthetics oils are supoose to be good to 500F before being hurt
On the blocks with fill I have seen a filled 340 block prepared by Greg Lunatic ( ) on the engine dyno at Pettis performance have 180F on the outside of the block at or below the fill line and 130 F above the fill line using a hand held infra red temp gun I had freshen it for the owners and built them a complete new R1 block non filled spare motor, guess which one made the most power