Main difference in -4 and -5 spec is that -5 has the friction modifiers already in it for p-trac clutches or cones, the -4 does not and you have to use the p-trac additive (graphite or moly based) for your brand of differential to stop snapping noises at cornering. You can run either one in open diffs.

When at parts store when they bought -4 and came back complaining about p-trac snapping, change to -5 and problem gone. I did it several times with different customers and spoke to the lube rep about it, he verified it. We carried the separate additive as well and sold it if they wanted a -4 lube. Manager (a woman) first clued me into it, not that I didn't believe her but I like more confirrnation. It seemed to be true. I had never heard it before either.

After some rooting around it may be that GL-4 had some of the additive but GL-5 has even more. Another reason not to run it in MTX, the synchros will try to slip more and not wanted there.

GL-5 works fine in a rear end all day long.

Is GL-5 OK for Pintos??? My repair books say to use 90w API GL5

VERY good article Wittsend! I really think Red Line MTL is the best tranie / rear end oil there is. Hi $$ but you only need to use it once for a long time. Makes a Pinto tranie shift like it has hot butter inside. Good stuff!