You need to be using a Gl-4 oil so as not to eat up your synchros. At any rate I don't think that's right for a gear oil. Its expensive, but I run Redline MT-90.
Remember you have a ton of heat blowing off of the block right in front. I can promise you if your entire trans was really getting that hot you’d be having issues.
I’ve driven around with no cover over the trans tunnel and it starts to bake! I think you’re trans and the oil in it is fine.
What’s the ambient temp in your area when you are driving?
You need to be using a Gl-4 oil so as not to eat up your synchros. At any rate I don't think that's right for a gear oil. Its expensive, but I run Redline MT-90.
I wouldn't recommend run GL-4. Use a specialty oil meant for manual transmissions like the Redline MTL or MTL90, Penzoil Synchromesh, or a quality synthetic 10W-30. I ran Redline in my AX-15, and it was night-day difference between the NAPA GL4.
I am running either MTL or MT90 in mine (I can't remember which I wound up with). I do not get temps such as you are describing. Not even in triple digit outside temps.
As to engine temps/hot air through the trans tunnel, I have replaced all the shot factory rubber boots. If yours are thrashed as mine were that may make a difference. (IIRC I used a TJ trans tunnel boot in place of an XJ boot that was no longer available.)