It's not cheap fluid but I can get 80w90 mobil 1 in a5 gal bucket for about the same as a12 rp quarts.
It's more cost effective for me cause I can do tranny and diffs. Yeah.
Won't hurt them. Might be a fuzz stiff when shifting in the middle of winter, but on a high mileage tranny, probably do some good. Especially with Mobile1.