Use Dextron ATF.
I just looked at (3) different engine owners manuals regarding the older Borg Warner Velvet drives, all 3 engines using the same transmission, only Indmar states using a variety of oils as -OK- to use, the others specifically state use Type A only (AKA Dextron) I use Dextron III, IV, V whatever is on the shelf with Dextron in front of the number, but change it every year.
Capacity: 1.9 qts for A/S boats.
This is also the time to do the following;
Lube shift detent
check & adj. shaft alignment