1990 Supra comp with pcm 351 Ford.

Recently got fed up paying 120 an hour for minor maintenance. I’m not sure what is in the tranny, but last year my mechanic had charged $550 for impeller, plugs, wires, cap, points, trans fluid, oil change. He ended up telling me my transmission fluid was fine and my impeller as well and because things took longer than expected still charged me $570 without doing the impeller or the trans fluid. I took over this year and am using a marine filter for oil, 25-40 or 25-50 mercury semi synthetic marine oil, and I’m pretty sure I have 20w in the trans because the fluid is clear. It’s still clean, but I want to change it. I can’t find straight 20w anywhere and seeing as I’ve never done a trans fluid change before I don’t trust myself to get all the oil out to switch to atf. I use a mighty vac fluid transfer system for extraction. I’ve heard running the drive is bad out of water and I know I’d need several flushes. I’m either going to pay a shop to do the trans fluid switch or stick with an acceptable oil. I would prefer not to change the type of fluid that’s in there unless it poses a serious risk. I’ve got up around 800 hours on the boat. What is your take on the transmission fluid? Can you give me any tips of what not to do? Will anything substitute for the 20w? I can get 0w20 synthetic or sae 30