|Max Gear 75w-90 with Synerloc and he verified by detailed lab tests that gear oil is non corrosive to sensitive metals in manual tranny and can be used in manual tranny with GL4 OR GL5 spec.
The RP Max Gear PDS states non corrosive and GL4 OR GL5 for gear box is acceptable:
I did a several month corrosive test with 5 synthetic gear oils using copper strip in the jars. Result the RP Max Gear had no corrosion and no discolor on the copper but the others did to varying degrees. None had metal pitting. Granted the test was at room temp and higher temp desired but not practical for my garage test. The other oils tested were: Redline MT90, Valvoline 75w-90, Amsoil 75w-90 and Mobil1 75w-90.
My tranny was replaced under warranty in 96 @ 32.7k due to dealer using the wrong gear oil. I requested that the tranny come filled with factory fluid, it did and used it that way until I replaced the OEM gear oil with Redline MT90 in 99 @53k and 01 @ 64k then in 02 @66k with Amsoil 75w-90 to help shifting did not help. Started in 03 @ 71k with RP Max Gear 75w-90, best of the 3 and used ever since.
Changed RP several times since then and no ill shift effects by feel and the tranny shifts great and I am careful with the clutch and shifts. This is my experience of course others will vary depending on left foot, right hand and oil brand and change frequency.
Here is a interesting discussion from a seasoned mechanic/racer that has worked on many manual trannies and talks about his experience (@ about 11:14 min) with GL4 and GL5 oils as well as simple flow tests with many brands:
I do not worry about RP but can understand your worry.