Once the synchronizer matches the gear speeds and the shift collar engages, the lube won't make a difference. Slipping out of gear is a result of wear on the small male teeth on the input shaft that the synchronizer sleeve engages, and the corresponding female splines on the synchronizer drum. If there's any taper wear at all on those mating pieces, the sleeve won't stay engaged on the back end of the input shaft for direct drive. Both pieces will need to be replaced.
Jerry
