ATF. It's already filtered so I don't have to pour something from a fuel can (or old filter) in there and possibly cause problems, IMO it's better than having the pump run partially empty without the lubrication of fuel while it cranks to prime, and it doesn't have to crank for a minute or two to prime (post 617 engines).

Has to be better for the engine.

__________________

Gone to the dark side

- Jeff