I have personally seen oil temps peg a gauge the had a max temp of 345 F in a dwarf car running a Yamaha FJ1200 engine.

The engine layout was poor as very little air made it to the rear most cylinders.

Never had an engine problem as the oil was changed every practice/race.

It is all relative, oil will work at high temps, but depending on the oil, it won't work for long periods of time.

Don't forget, oil is tested at 100C for viscosity qualification.

Ken

So what is the consensus on the water thermostat? 140 or 160? When I ran a 160, my water temp got up to 175 and my oil around 190 - 200. This was in October and the water temp of the river was getting colder and I only ran it up to 4000 rpms or so for short spurts.

I was out this past weekend again with the 160 degree thermostat. The water temp got up to 175 degrees and oil got up to about 200 at 4500 rpms. I'm just wondering if I should go with a 140 degree stat? I think I read somewhere that the 140 is used in motors with thunderbolt IV and the 160 is used in motors with thunderbolt V ignitions? I have the thunderbolt V.

Doesn't make any difference...just under 260

I found that with my carbed motors they ran a little stronger with the 143º thermostats than they did with the 160ºs. With my MPI motors
the computers don't seem to like the 140º and it actually runs a little stronger with 160ºs. Your oil temps are in the sweet spot.
As it only takes about 15 minutes and one gasket try both and let the engine performance make the decision for you. JMO