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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?
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