My first thought, based on how you said you ran it, was the same as GT Jets, oil temp.
If you have a large capacity pan, being a quart low should not be an issue. Oil temp can be caused by extended running at higher RPM and load. If you made a lap at Elsinore at 4500 RPM and up, it's basically the same as racing it. Personally, I like straight weight oil for "racing", rather than multi grade. I know the Valvoline is the easiest racing oil to get your hands on, but in my opinion there are better choices.
With 600 hours, if you have been running it that hard all of it's life, it could be getting kind of tired?