The stuff Steve is talking about is probably the synthetic manual trans lube
that the over the road trucks use. It's considered a 500,000 drain interval in
OTR, and lifetime fill for light duty. For what you're driving, you might just
take Mac's tip on the Mobil 1 15w-50 synthetic motor oil. More than one person
has run that in IH manual trannies and transfer cases and been impressed with
how much better the old gearboxes work with it in there. You need to make sure
all of the old GL-5 type gear lube is gone first though; an easy way might be
to drain and refill with some regular old dino 15w-40 diesel oil, run it a
couple thousand miles, drain, then do the Mobil 1 thing.