2nd gear won't engage when hot
 

First day on track with the car (at laguna seca) and while the weather is awful today it's a good enough chance to shake out any major issues with the car.

After the first 20m session, I noticed now my 2nd gear refuses to engage once I've warmed up on track. Once the car cools down a little, 2nd gear works perfectly again. All other gears (well at least 3 and 4, don't really use 1, 5, or 6 on track) seem to work fine.

It doesn't grind or anything, just doesn't "notch" into gear. The shift throw is about normal, just no engagement

Any ideas? I just changed transmission oil last month fwiw, my gut feel is either transmission temp getting too high or shifter/cables/ends are too worn (all original 20 years old)

What oil did you put in it?

Mobil synthetic Delvac 75W-90

Chatting with a few other track buddies, they seemed to think that the Delvac might not be very good at handling high heat, and recommended either Motul or Redline gear oil. I don't mind changing the oil again as it's pretty easy, but I'm trying to make sure I understand why the oil would cause one gear to not want to engage. It was obviously related to RPM/heat and started working again as soon as the engine cooled a bit (literally right after 1 cool down lap, 2nd worked again fine) so I guess oil temperature could make sense?

To me that would indicate a worn 2nd gear synchro. If the oil isn't doing it's job - it's just making it worse.

The OE factory fill is best lube by far.