I would buy like 18 quarts and do a drain and fill (like aredub explained) 3 times this would seriously minimixe the amount of 'old' fluid in the lines but if you use a expensive brand (i.e. Redline, Royal Purple, B&M, amsoil) it could get expensive.............

PAUL! 'looking for someone to T-Tec in some Royal Purple Trans fluid'


..... a little on Royal Purple Oil's........
it is a little known fact that Royal Purple is the WIDEST used oil in professional Racing. Due to sponsorships people have been know to 'sponsor' others and then use only Royal Purple. It is superior. (IMHO) i think the whole thing is kinda funny but a very reputable NASCAR engine builder at ENDYN (do a web search) let me in on this. So i have to give it some merit.......FWIW