What do you put in the rearends? The Trans hyd stuff I see is $70 for 5 gallons

Rearends usually take 80-90W gear oil.

Are you looking for guys that know old tractors, or old guys who know tractors?

This old Massey 44 had hydraulics but it's all unusable now so I think 80/90 for the gears would work okay.

on an old tractor, you can usually get away with putting in whatever because they're not as picky on the fluids (since everything today is so much better).

However, the hydraulic fluid for the hydrostrat on my tractor is checked and added to the "rear end" and if I put the wrong type it would quickly ruin the hydrostat.

To play it safe, I'd suggest selecting an appropriate subforum on tractorbynet.com and asking there.

If by old you mean 40's give or take a decade when they had large gears and piston hydraulic pumps in the rearend probably 80-90 although I've heard of someone using 40 weight in his Fergusen
I would say the rule of thumbs would be if it has a gear type hydraulic pump they wouldnt standup to the heavy oil especially when its cold