I had an interesting conversation with an oil guy (who knows more than I do)... he thinks that overfilling the NV5600 is causing "hot parts" (bearing and gears) to recieve more lubrication and cooling from the oil. And that is what caused my guage to read hotter! It makes sense to me, so I'm going to overfill again on this attempt.
Interesting subject - especially with the related lube level issues the G-56 had/has. I have always slightly over-filled my 5600, but only by about a pint - as much extra as I could get in by parking the truck on a slight incline
When you speak of overfilling yours, what amount of overfill are you considering?