Start with the 49 shop manual's recommendation (attachment).
Today's oil can have additives and too much advertising. They range from snake oil to somewhat beneficial. I have read about synchronizer wear due to sulfur content (GL5). I've also read that "slick" oil can make better shifting but can be bad for old style/design bearings. Then there is conflicting opinion/comments of super XYZ brand that has run a million miles on their trucks and cars.
I like plain conventional 90wt. found at your FLAPS.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
