It the truck was driven much without lube in the transmission the bearings are suffering from excessive wear. If I were you I would rebuild that transmission before you have a catastrophic failure it is getting tuff to find gears for some of these old transmissions. You should be able to find some one to rebuild it for 4 to 6 hundred if it’s just bearings and a synchronizer ring or two but may become very expensive and near impossible if you damage any gears or shafts.
I concur with Wrenchbender lubrication is preventative not a cure no oil will fix damage.
