Hi Thomas,
If the ISO 32 fluid that Rusty mentioned is difficult to find, you can also use SAE 10 non-detergent motor oil in the gearbox, and to top-off the Fluid Coupling.
The ISO-32 Tractor Hydraulic Fluid does have anti-foaming agents that make it better-suited for use in the Fluid Coupling.
Good luck !
_________________________
Frank McMullen
1941 De Soto, 1948 Chrysler NY,
1941,1954, 1955-first, 1989 Chevy trucks,
1960 Chrysler Windsor,
1964 Valiant Signet Convertible,
etc, etc...