Oil for old tractor transmission

I have an old tractor, a 1962 Massey Ferguson 35.
Since the hydraulic pump is somewhat worn, but not worn out, what would be the best choice for transmission oil?
Here's the parameters I have to use.
1. It uses the same oil for the transmission, differential, and hydro pump. It's all one big oil sump.
2. The original spec called for a 90 weight straight mineral oil with no additives due to the use of yellow (brass/bronze) metal parts.
3. Since the hydro pump is a little worn, but still ok, the modern TDH (Transmission/Differential/Hydraulic) oils that newer tractors use are a bit too thin for summer use, but work great in the winter.
4. The 80w90 GL-1 in there now works great in summer, just what the hydro pump needs, but is a bit too thick in winter.
I have heard about 15w40 engine oil being used in it, but I don't know anything about that.
What do you think?
Original Post
Consider the use of 5o weight transmission oil for use in heavy truck transmissions. early 13 speed trannys used mineral oil but later specifications use 5o weight motor oil. delvac makes an excelent transmission specific 50 weight that i have used for 950000 miles in my eaton super ten. I belive that 50 weight motor oil and 90 weight gear oil Viscosities are the same. Good luck.
Hello Old Farmer,
The 80W-90 will actually have a better low temperature start-up than what the origional SAE90 would have. The warnings regarding the usage of EP's was lifetd by AGMA a few years ago, assuming certain conditions were met ie less than 90 degrees C. This of course assumes that you are using an oil from a recognised OEM blender. If your winter ambient temperature conditions are less than -10C or so then you might want to consider something like a 75W-80 gear oil (~47 cSt @ 40). Or if you think that you just need a lower start-up product then you could consider a 75W-90 product. These are avaliable from most of the major NA suppliers.
An engine oil like 15W-40 has a vis @40 of around 110 cSt @40. Again if you have start-up temperature concern issues you could select a 0W-40 product this would lower the pour point to very low temperatures -35C (you probably wouldn't want to be operating yout tractor at these temperatures)

hope that helps

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