As with engine oil, I only trust Castrol products. I use Castrol Dexron III/Mercon in all of my cars and it works just fine. If you are changing your own tranny filter and fluid, make absolutely sure that the cork gaskets come off when you remove the filter or else you're looking for trouble. After you have the new filter installed and the pan back on, fill with four quarts then start the car and let it warm to normal operating temp. You can then begin to add fluid to specified level. One more hint, after you have topped the fluid up, put the car in reverse for about 30 seconds or so, this will clean out the valve body. You can then cycle it through the rest of the gears then re-check the fluid level. Hope this helps!
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'93 300E 2.8