OK, I understand the importance of using GL-4 vs GL=-5 gear oil in the Muncie manual four speed transmission found in my 1968 GTO...protection of the yellow metals, right? But...how about the rear Safe-T-Track (posi) differential? Paralysis by Analysis is beginning to set in...I have read that GL-4 is not appropriate for the rear, rather GL-5 should be used. I'm wondering is GL-5 was even around the scene 50 years ago? The factory service manuals I have make no such reference to GL-3, GL-4 or GL-5. Seems to me GL-4 should offer the required protection (with an anti slip additive) Very interested to read everyone's thoughts on the subject. Thank you in advance!