John we ran SAE10 in the transmisson, anything heavier and you will knacker the gearbox.
SAE 10 is vital due to the fluid flywheel, it has a lock out system for over run and for when engine revs hit a certain speed so the transmission becomes a full manual, then if the revs drop down, like climbing a short hill, the fluid flywheel will unlock allowing the engine revs to pick up without chaging gear, alomst like a splitter box. if the oil is too viscous it will not work properly and you can burn out the fluid flywheel.
We had no problem getting hold of the oil from a local "76 lubricants" shop in Azerbaijan, but they did have to import it for us.
i attached the gearbox pages from the english manaul. :-D
Baz