I use whatever is on sale when I'm out of wash. Currently I'm using the Armor All carwash concentrate, only because I found it on clearance at Target for 49 cents for a big bottle! Seems to work fine. Previously, I have usually used mild dishwashing liquid and have never had problems with it, although some people swear that dishwashing liquid is bad for paint/wax/clearcoats. But I've used that stuff forever on different vehicles with no ill efects.
I agree with DaveDatsun that water quality has more effect than soap though, for a good wash.

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2001 SE 4x4 5-speed
Super Black