Re: WD Hydraulic fluid
Inwywind, you should use the Universal Hy-Tran (hydraulic-transmission) oil. In some old machines, the transmission, hydraulics, and differential all used the same oil out of a common reservoir. I think your WD-45 is like that. I know the B/C/CA is. Ford and Ferguson used this system with 90wt. oil, but they used a big piston. Allis used small hydraulic pumps in a high pressure system, and specified the thinner, 20wt. oil. Nondetergent was used because it did not foam and the detergents were unproven in that era. Nowadays, the UTF is the accepted substitute and it has additives that 20wt did not. It is available from Tractor Supply, or Case-IH dealers (Hy-Trans). Other dealers probably have it under some other name.
Don't use regular hydraulic fluid. It works in the hydraulics, but doesn't have the viscosity you need for the differential. The final drives still use 90wt. I've heard of people using 15-40wt in Farmall Cubs, but I don't know if they have heavy drawbar loads or just mowing. I don't know how it would hold up in a larger tractor, but I would think any modern oil would be better than 20wt of the 1940s.