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I had been using 3ozs of Walmart TC-W3 - didn't seem to do much for my MPGs, but didn't hurt.
Last 2 tanks, I started using 4ozs instead - it does seem to yield better mileage (notwithstanding our switch to winter blend, which lowers fuel efficiency).
My understanding was that TC-W3 would also lubricate and clean cylinder walls or something (as per bobistheoilguy) - and in my old Celica, it did improve my MPGs by 1-2 (for a ~30mpg car).
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