I'd put what GM wanted in it, JMO-closest now available is 85w90. The usual shifting troubles w/that model truck are the column & linkage-usually old sticky grease in the column or worn shift ears. Again, JMO as a 30 yr mechanic.
He didn't say if it was a 3or4 speed but I gather by the amount of grease it must be a 4 speed. I agree if I built it myself I would put in the 85/90.
