Every 25K in the tranny, since you are using a good old 30 wt...
if you have Royal Purple elsewhere, I don't see a reason to ever change it...
200K plus on Mobil Synthetic 75 W 90 in my 4 Runner's trans, have given zero problems....and actually last time I checked it, I was surprised how clear and new it still looked...
clear and new doesn't necessarily means that it might be perfect, but its doing a good job and I don't believe it needs a change...
