I had a 1996 3.8L Windstar.
Owner's manual specified 5W-30
TSB later recommended 5W-20, at which point I switched to the 5W-20.
I ran that oil for most of the time I had the van, got it in 1999 with just under 38K miles on it and sold it this past spring with over 229K miles on it.
It was still running VERY well.
I would recommend the 5W-20 oil year round.
It is VERY important to stay on top of transmission fluid changes.
The transmission fluid needs to be changed every 30K miles for normal service....that is a full fluid exchange, replacing all the fluid.
Some folks do a drain and refill of the pan, but that would need to be done MUCH more frequently.
I also installed a auxillary transmission fluid cooler.