The only way I would use used atf is if I were to refractionate it some how. That would eliminate all metal particles and dirt in it. Seeing that its no small task to do so, I don't think its practical. If frenchfry grease had metal particles in it I don't think anybody would be doing bio diesel.

Use all the new ATF you want. Every time I change my fuel filter I fill it up with ATF before I put it on the car. The concentrated slug of it really cleans things out.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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