Help understanding gear oil/hydraulic oil ratings

Folks asking for knowledge/advise on understanding gear/hydraulic oil ratings and what I can use in my 1964 Ford 2000 4 cyl gas engine w/4 speed trans. Original manual listed a mild extreme pressure lubricant SAE 80 for both trans and diff and a M-4864-A or M-2C41. Now recommendations have changed for these old tractors and from what Ive read/be told to just run a good universal hydraulic fluid in all three to prevent contamination of using 2 different oils and have leak. Recommended UHF by Ford is M2C134D or equivalent. Well I know I got leaks (trans and now possibly diffworking on it) however leaks not bad yet so until I get smart on how to split tractor and replace seals will run as able.
Long story longer looking to get gear oil 80-90W for trans and diff but see it comes in ratings GL 1-5 mostly with 5 being most common. Keep reading comments to be careful of GL5 gear oil as harmful to yellow metal brass and should not be used in older tractors. Does my 64 Ford 2000 have yellow metal? Also the gear oil sold at tractor supply and Agrisupply also have GL1 80W-90 and 80W that is cheaper than than the GL5 stuff. Is it just as good for my purpose or will I wreck my tractor with the cheaper stuff? Any advise/knowledge is welcomed.

perryjosh64,
I can only tell you what I have done on my 1964 4000.
I use Shell UTF that I got at TSC for all three compartments.

I do not like using the older style lubes in the tranny. Too much resistance at cold temps. You don't need it in the rear end either. Modern lubes are much better than the old gear lubes.

I only use a Gear Lube in the gearbox of the brush mower.
YMMV
Keith

Like Keith said. Use a modern UTF in all 3. The modern UTF's have Extreme Pressure (EP) additives that make them good for use in geared environments like transmissions and rear axles, and they are a lighter viscosity than older style gear oils that can be fairly slow to flow in cold temperatures. They have friction modifiers as well for use with wet brakes and clutches but you don't have to worry about that with your tractor.