If the oil (I dislike calling oil a fluid like it's a magic liquid) hasn't been changed for this long, and especially if the truck doesn't have an oil cooler thermostat bypass, I can assure you there are internal deposit issues clogging things up.o
Get about 3 more filters, including the main flat filter, and a 5 gallon pail of MOPAR ATF+4,
and change every couple hundred hours. If it doesn't clear up, you're screwed and need an overhaul and cleaning.
Hate to say it but im thinking this.
Iv been very absent on the forum but i can say years ago I was heavy into trans fluid research, alternative trans filters options (stock sucks b@!!s) the metal thread vs plastic on and on. Cutting brand new filters open and stock ones. Bla bla….
Biggest….BIGGEST most important thing EVER for the RFE…. Not filter choice. Not metal thread thingy.
#1). Keep it clean.
Only way is changing the fluid……i mean…OIL .. very frequently. The RFE is actually a pretty good transmission minus programming.
Her down side is she will NOT deal with anything but clean fluid. I wont go into the oil debate any more. But i do not use ATF+4 and will not. I changed fluid every 20,000 and filters every other fluid change. Its not crazy. Its a clean trans.
To may known RFE trans failures/problems from dirty fluid.
I remember a fella (passed away) that ran valvoline max life trans fluid in his 8 speed. Heaven forbid maxlife in an 8speed. He changed it often and it ran great.
and Mike nailed it too. Get the metal thread thingy anyway. Just return it if the threads are sloppy. Check it on a new filter. The one Mike gave you a link to is a good one in the sense that the threads are nice and tight fitting as well as the center is broached for an Allen wrench. Makes it way easier to get it on and off the filter and trans Housing. Imagine trying to unscrew that thing with a straight thru hole in the center and nothing to put a wrench on.
And before you drain it again get. UOA kit and send in a sample NOW. Check the TBN too. $30 with postage give or take.