If the oil pan is easy to drop, get an OEM gasket and drop that sucker.
Otherwise, at only 48k, a few simple drain n fills will work like a charm. depending on how dirty the tranny fluid is, i'd just go directly to your oil of choice, in this case, Amsoil.
I'd do a drain n fill with Amsoil, drive around for a week and then do another drain n fill. This will insure that the oil concentration is as close to being 100% Amsoil formula as possible which is better then having say 50% of some cheap fluid and 50% Amsoil if you were to do it the way you described.
Just my :2cent: having some experience and good results doing it that way (and every 60k or some after that)