Fast road/track use gearbox oil

I have redline MTL / ATF in mine, does the job. Didnt notice a massive difference over the old stuff, however gives piece of mind there is quality oil in there.
 
I'm looking at replacing the gearbox oil and from the looks of it on here, 75W-80 is the one to go for. The box is quite old and has started to crunch into 2nd when the car is very hot (maybe oil boiling?) Rang a local garage to see if they can change the oil for me and put in 75w-80, they rang Redline directly who have said 75w-90 is the best one!?
@raptor676 how do you find the 75w-90?

Cheers