You may be confusing the oil to use for the primary versus the gearbox. The pre-unit BSA clutches run in a small amount of oil and most use Ford Dextron ATF in the primary. You want to use a gear oil in the gearbox.
Re: ATF vs. 80w-90 gear oil for gearbox?
#533739 03/19/1411:11 am03/19/1411:11 am
Regardless of if you have heard that it's been used, it's wrong.
While at work I broke out my Viscosity Equivalents chart, ATF is nowhere even CLOSE to the viscosity that the OEM had recommended (50 Weight Engine Oil). Times have changed and now we have dedicated gear oils that have a host of additives that make it ideal for our application (straight cut gears).
The correct cross to SAE Grade GEAR OILS, is 90W (Kommandos suggestion is fine). If you google it, you can have your own Viscosity Equivalent chart. I find it useful as I'm constantly converting AGMA to ISO grades to MobilSHC series numbers.