Bottom line is you fill it to the proper level, whatever it takes. Most tranny are somewhere between 7 and 9 quarts. The 126's are straight forward. No magic here. Just drain the torque converter and pan, new filter when appropriate, and refill. To refill, put 1 gallon in, then run the engine. While it is running put the rest of the fluid in. This will keep you from overfilling the case when the torque converter is empty. Fill to the mark whatever it takes is what it takes. One problem when draining the fluid using the drain plug on the pan is that it leaves about 3/8" of fluid in the bottom of the pan. Probably about a third to a half of a quart or so. Removing the pan gets it all.
As far as fluids are concerned, its my belief that Dexron III is Dexron III. It does not matter who the vendor is. I buy mine at Kmart, $5 a gallon. I can just feel the reaction of some of the members as I am writing this. It is probably all made at the same place. Correct me if I am wrong. Now I know there are many different synthetics out there but Im not talking about them.
Just my opinion.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k