1990 Supra comp with pcm 351 Ford.

Recently got fed up paying 120 an hour for minor maintenance. Iím 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 Iím pretty sure I have 20w in the trans because the fluid is clear. Itís still clean, but I want to change it. I canít find straight 20w anywhere and seeing as Iíve never done a trans fluid change before I donít trust myself to get all the oil out to switch to atf. I use a mighty vac fluid transfer system for extraction. Iíve heard running the drive is bad out of water and I know Iíd need several flushes. Iím 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 thatís in there unless it poses a serious risk. Iíve 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