Lee........I'm so glad you asked 'stedda' searching the archives at the top of this great N-Ford forum.
We do NOT recommend GL-5 'cuz it is NOT brass friendly. There are brass parts in yer tranny and hydraulic pump.
While the original hydro-tranny oil was spec'd as 80wt gear oil in 1939, ittza kinda rare and hard to find. HOWSOMEVERS: GL-1 is a close approximation. Ford/New-Holland now specs: MS-134D hydro-tranny fluid which is about 60wt. Many complain that the THIN 60wt does not do well in worn hydraulic pumps and cylinders. Yer call, I use 134D ........Dell