I use 15W-40 in all my Dextas and Super Dextas. While the specs call for 30W, the Fordson forum concur with 15W-40.
The debate on oil in rear in and transmision however will be hot and testy.
The specs call for 30W in each, but Ford posted a bulletin permitting ford hyd fluid in rear ends. I use Ford hydraulic or equivilent, largely because all my Dextas have remote hydraulics and I plug right into the tool I used on the last tractor, and there is a bit of fluid sharing this way.
Since I live in VA, where it is not super cold, so I use 90W in tranny and steering gear. The original spec bulletin called for 50W in tranny, but later bulletin said 30W. so - straight 30W or 15W40 in engine.
IMHO - if you choose an oil and stay with it, you'll be fine.