Tractors up to 1957 used 90W in the gearbox and hydraulics. In 1957 Ford changed the specs in a service letter to use 30W or 20/30W Universal in all tractors, gearbox and rear axle, so both grades are correct.
Do not use 90W in the hydraulics of Fordson Super Majors or any of the Dexta range from 1957 as the porting in the hydraulics is much smaller and the thicker oil will cause problems. (The early Dexta handbook says 140W in the hydraulics and rear axle but this is an error and was changed in later print runs).
I still use the 90W oil in my 1952 Major but the Universal oil in my Dexta.