unfortunately there are alot of variables to tell you why. A few things. though you ahve a 2qt accusump that oil is for the most part stagnet, and not moving when the engine is running. So that doesn't play a major role in cooling. Teh oil cooler definetly does. An oil temp of 240 isn't that high. Any decent synthetic oil will handle that no problem, strickly from a temperature standpoint.

Is it possible to get an oil sample to send to a lab? did it smell? dis-color? etc..

What oil was used when it was actively raced? are you running large clearances? tight clerances? you have alot of OP cold but what about flow? Does it have the stock pump?

We tired a few oils and found what worked for us. Racing oils will help disipate heat better, they also typically have other additives that will no allow them to meet API certs.

There is a decent read, though you may have to pay a few bucks for it. look up the "oil Bible". It is a few years old but the info is good.

Another thing you can do is do a standard viscosity test with the oil hot v.s cold. There are stardard tests you can find online to condut to see how it is changing with temperature. IT should change. hence the 20w50.