IMHO, I would never use an alternative transmission fluid, automatic or manual, unless it was a direct replacement or it supercedes the OEM fluid specifically. Some companies use additives or detergents in their fluids. Those of which could be harmful to the seals/parts in the transmission unless they were designed to use it.

Another thing to is that not every transmissions have the same tolerances. Some people may think that having a higher viscosity fluid would be better because it's thicker and would stick better to the parts that needs to be lubricated. However, if the transmission has tight tolerances, thicker oil will not be able to work it's way into those areas. It also works vice versa, if the viscosity is lower, it may not lubricate it enough.

 

I've been using the Amsoil ATF for several Gs lately and it has been performing very well.

 

I just did a drain and fill with S matic. If you drop the rear of the car down, you can get 4qts out.

 

S replaces J. using S would be perfectly fine.