I was running the NEO RHD cocktail that English likes when I shredded a fresh 5 speed. The car made 557whp, 455ftlbs on the dyno and it was turned down a couple pounds for street duty. I won't lie, I beat on that tranny, it lasted 6,000 miles or so. I ripped 3rd apart, which then took out the rest of the tranny.
The car is back on pump gas, making 440whp/335ftlbs, so it's a lot safer, but I'm still a bit worried about the longevity if I decide to take it to the track.
I run MT90 now. It's easy to get and is a bit cheaper than NEO. I don't think MTL, MT90, NEO, or anything in the correct weight will save a tranny. It would be such a thin line being toyed with, it'd only be a matter of time until the gearset is fatigued and lets go. Synchromesh won't offer the protection, and heavyweight can send paraffin into the oil ports and may have adverse affects on shifting, although I never did try something this heavy in my tranny. I don't think even the heaviest oil will be the saving grace.
I dunno, some people can get away with abusing an old stock transmission for years, where others are breaking HD gearsets in no time.