Gordon,
The oil in the six-speed is quite thin, similar to 5W30 motor oil. The Mopar specified lube is quite expensive. I use Amsoil 5W30 heavy duty diesel engine oil in mine. 22,000 miles and no problems. Shifts smoother than stock in all temperatures. I took and oil sample and sent it in to be analyzed on 1/14/02. I will post the results as soon as I get them.
Others are using Redline MTL with good results.
The transfer case takes ATF, the axles take gear lube, so there is no one-fits-all lube.