Offhand, I believe the interval for tranny fluid replacement is about every 25,000 miles. Your manual should say. More often can't hurt and I think Tone once said he does it every 15,000. I did the differentials myself with Royal Purple and have no complaints. Like you, I wanted to go with it in the transmission as well. I offered to supply the fluid but the place I used said that the machine they had was set up on Mobil so I reluctantly told 'em go ahead. Really wanted to run Royal Purple there too but since it is a very reputable shop around here I gave in rather than take my chances elsewhere. I had them drain, flush and refill, and had the filter replaced. Make sure that the filter you get comes with gasket made of something other than cork. A simple drain and fill doesn't replace all of the fluid or force as many contaminants out.