I don't know if this will apply or not, some may disagree with using synthetic oil of this weight in that style transmission. I use the 80w-90 in my 2 ton.
Anyway, my son has a 1986 Nissan Sentra 5 speed manual. It wouldn't shift into second so I took it to my mechanic, whom I trust a great deal. We decided he should pull the transmission and send it out to a rebuilder. Before he did that though he consulted with the guy and he advised to drain the oil and re-fill with GM's special synthetic for manual transmissions. That solved the problem and that transmission has been shifting like new ever since.
(from GM's web site)
GM Goodwrench Synthetic Manual Transmission Fluid GM P/N 12346190 or equivalent SAE 75W-85 GL-4 gear oil
Did you try your truck' engine for an oil pressure reading? Hope it worked out well if you did. I'd love to drop by and chat with you but I'm 1325 miles northwest of you, in Winnipeg, Manitoba.
Nice to see you on here and to be getting some answers.
Cheers, George
