I've used Bio, gasoline, ATF, motor oil (all in one tank too) in less than 30% ratio to regular diesel. It will run on it, no doubt. I agree, though, that you'd be better off to filter it down and maybe cut it with something thinner to get the viscosity close to diesel. I'm told Bosch pumps on old Mercs tolerate higher viscosites better than rotory pumps. I wouldn't use it if its been sitting exposed to air(humidity) either. I just don't know if ATF is water retentive like motor oil is. Trannys have screens mostly not paper filters like the engines(except for Subarus as far as I know). I'd want to filter used ATF better were I might be inclined to pure used motor hot from the sump to the fuel tank since its been through the oil filter and will be going through my fuel filters before it hits the injectors and pump. Also if you react used motor oil with you biodiesel it will precipitate out the particulates between the methyl ester and glycerine layers. Maybe ATF will work the same way. Its worth investigating.

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'83 300DT
'04 Forester XT--modified
'61 Beetle EFI Turbo
Burley recumbant