I started to post this in the discussion about engine oil but decided a new thread might be better. I recently had my gear shop put in a 3.25 gear/traction lock chunk in my car and while they were putting in the gear oil, I had them fill the transmission, a 4 speed Toploader. They probably added a quart or so. Driving home the car didn't shift right. Long story short after a little research I discovered that GL-4 80W-90W gear oil was specified and GL-5 WAS NOT to be used. Well GL-5 is apparently the new spec so GL-4 is getting very hard to find. I would up putting MTF (manual transmission fluid) in the car and it shifts normally now but the MTF is very light and is recommended for GM and Chrysler manual transmissions, not Fords. I ordered GL-4 Joe Gibbs gear oil from Summit, but I found this very strange so I thought I'd share.
Good thing you removed the GL5, it has additives that will eat brass parts ( synchros ). I think GL5's are synthetic. I'm using it for the first time in my rear end.. Even finding GL4 thats not EP rating is getting hard. The last I bought was Sta-Lube from Amazon that claims it won't harm soft metals.
PS. Synthetic also causes the syncro to stick making shifting hard.
As highball said, the newer GL5 is not compatible with the older ''yellow metal'' syncronizer rings, most modern transmissions use either paper lined or other non metal based surfaces. And, yes, the GL4 spec fluid is getting harder to find. In my 59s 4 speed Toploader, I installed Brad Penn GL4 fluid. It is not super easy to find, I buy mine from Ken at Oregon Cam, where I also buy my Brad Penn high zinc engine oil, as well as most of my camshafts.If you are not in the Northwest, the Brad Penn website has a dealer location list.
I looked for Brad Penn but Summit and Amazon say it is unavailable. David Kee Toploaders recommends it. I will look at the Brad Penn website for a dealer list, thanks.