First time I changed transmission oil in my '48 P15 I used whatever was on the shelf at the local auto parts store, think it was GL4. Then the shift from 1st to 2nd with a cold trans got crunchy. That is when I did some homework and found the modern GL oils are too "slippery" for good syncro ops. In spite of being skeptical of "repairs in a bottle" the switch to RedLine instantly solved the shift issues, it is now smooth at all temps. Knowing the syncros are also happy is worth the extra $$$$'s to me.
The diff has GL4 oil, no brass parts or syncros to worry about there.