||gear oil like GL2 thru GL5 are downward compatible ONLY for rear ends that were originally spec'd for lower GL ratings and can usually (if no brass internals) use GL5 (straight from Mobil tech department).
If the multi rated container specifies for GL4 manual trannys then ok to use. I have tried several gear oils in my 5 speed like: Redline MT90, Valvoline 75w 90, Amsoil 75W 90, Mobil 1 75w 90 and all gave shift me problems. Until I contacted the oil tech department as noted and got the correct meaning of the GL multi rating. I finally settled on Royal Purple Synerloc 75w 90, it is rated for GL4 manuals and GL5 rear ends. I have been using it for 7 years now and runs great for my tranny. . Manual trannys can be finicky like women ;-)