I have to go to 2 or 3 places to get 12-13 quarts of Mercon V of matching manufacturer for each of my 4r70w cars. I like to be able to find 13 quarts of Castrol, but the last two times I have had to use AAMCO. I don't remember who makes it, I think Exxon.
I change mine once a year, every 20k miles, in the Grand Marquis, along with the filter.
I don't put many miles on the 97, only around 11k a year, so every other year for it. The 89 gets Mercon whenever something happens that requires a fluid change, usually the two small sections of soft tranny line near the radiator bursting or rotting from the heat. I can't find the Motorcraft memo on stopping from licensing Mercon to aftermarket, I assume the 89 will be off the road before that happens, or I will go to a universal oil. I still don't know what that means for the 4R100 crowd.
That would get expensive, 8 bucks a quart at 13 quarts, but a new tranny is a little bit more, and a lot of work. I am debating on if I want to go with it next fluid change for the 97 Cougar, 110 bucks in fluid over three years isn't a big deal, 110 bucks a year is.
But I go the other way in the engine oil department since it isn't a big price difference, the 00 Grand Marquis (I bring it up because it has the same basic engine/tranny combo, nearly the same weight, etc) gets the extended oil (15k miles) so I have one oil change a year (and one add up quart of oil), but the Cougar gets the regular 5k stuff, or Motorcraft semi synth when I can get it for 2.25 a quart (at Wal-Mart very infrequently for 5w30, usually only have 5w20).
97 Mark VIII LSC/Silver/Gray leather
97 Cougar 4.6 Sport/White Opalescent Tricoat Metallic /Gray leather down but not out?
89 Cougar LS 3.8/Oxford White/Gray velour/HG need replaced AGAIN
00 Grand Marquis LS/Vibrant White/Gray velour
63 Falcon 302/black and rust/shed isn't moving
82 F-150/Oxford White/dumb VV carb doesn't pass emissions