Amsoil is good stuff no doubt and comes with a cost. Originally the Marks came with Mercon. Mercon V is substantially better than the original Mercon (stay away from Mercon LV for these cars it is different yet).
If you were to have driven my car when I got it at 31 or 33k vs now at 38k...
Most of the transmission ‘act ups’ on these cars are due to the fluid. Drain all the fluid: pan, Trans, torque converter and refill with Motorcraft MerconV and throw in a new filter. You will be surprised what just MerconV does for these transmissions.
You are exactly right. I lay old towels down wash the whole underneath with brake cleaner after the fact. I used a whole can just on the k-member/frame, the the side of the block and under where the oil filter is. What a mess.