atf-4 is the recommened fluid and it is true other fluids as in dextron will affect the shift quailty however I have seen 604s go for years with the wrong fluid in them BUT I would use the correct fluid.

70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4