I actually got into a pretty good discussion with the boys at my own local NAPA when I went in to buy some GL4 last summer - they didn't realize that GL5 isn't an UPGRADE to GL4, it's just a different specification... and they didn't know that GL5 (with its large dose of moly-d) actually slowly destroys the copper-alloy parts in a synchronized transmission. They now know me pretty well.
As far as I know, moly-d is by far the bulk of the EP "additives" is GL5, whether GL5/MT1 or straight GL5. It makes sense for that to be true, since moly-d has fantastic lubricating properties on its own... similar to graphite. I use it in the machine shop for way lube on the milling machines & metal lathes.
FV911, you're right on the number about the temperature being extremely high right at the point of contact between two gears. All the power of the engine is focused at that point of contact, so there's fantastic pressure there - even if only for a moment per revolution. That pressure generates a lot of heat, leading to high temperatures. Heat is generated in the syncrhos themselve, too, because they're designed to work through brass-against-steel friction; they "scuff" tight" and "scuff" loose again every time you change gears.
Speaking of which... the addition of moly-d can also interfere with the proper mechanical action of those synchros even disregarding the fact that they're made of a copper alloy; gear oil with moly-d actually has too much lubricity for the synchros; the (correct) GL4 isn't as slick. That's why so many people who switch to GL5 (because GL4 is harder to find, or because they think GL5 is automatically better) wind up grinding gears from time to time - their synchros can't work properly.
My own final conclusion (whether or not it agrees with anyone else's) is that if your truck isn't much longer for this world and if you regularly subject it to really severe conditions (more than the Army would have subjected it to when it was new)... then use GL5 (MT1 or not) because of its ability to withstand vast abuse. Otherwise... especially if you (as I do) intend to drive your truck for a great many years... keep buying GL4. It's not very popular so it's a little harder to find, but it IS out there. Pennzoil still makes it (part #56076) and Amsoil still makes it. You can buy the CRC brand on Amazon (about eight bucks a quart). Don't even get me STARTED about those "color" (purple, red) gear oils...
