I switched to Mobil 1 synthetic ATF at 90,000 miles, mostly because my car sits outside during our usually very cold winters, and synthetic tends not to thicken as much in really cold weather.

I'm not sure you'll get much bang for the buck, unless your car is exposed to cold. You've obviously done a great job maintaining this vehicle to have 220 k on the transmission!

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Steve
'93 400E