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.