Sir, My predecessors wrote the manual you are referring to, you see I was employed by Ford UK as a engineer from 1959-1978. The manual does state that a 20w-30 is suitable but please remember that Ford had adopted the 90W standard as suitable for all models.
Service letter July 1959 states: Gearbox, Rear Axle and Hydraulics oil specifications have changed. Now all models from 1952 onwards use: S.A.E 30HD or S.A.E. 20W/30W in all the above at all temperatures.
Gearbox .. 4.5 imp.gal .. 20.43 lt Less PTO .. 4.25 imp.gal .. 19.32 lt
Rear axle .. 9 imp.gal .. 40.91 lt
I prefer to use the pre 1959 oil specification of 90w in my Major collection, but you may chose to use the later alternative.
Respectfully, Graham Barrington, Stamford England