The pushrod that goes through the transmission, should be removed cleaned and lubed, I don't know if it really matters what is used, as I used gear oil for 20 years, with no problem, a high quality grease would work as well.
I have an '81 model, so I'm not familiar with the bearing at the back of the transmission that the rod goes through on a '79 model, they are different, but it should be cleaned and lubed as well.
While you've got the bike apart this far, you may want to think about replacing the rear main seal, and the oil pump cover o-ring.
As far as gear oil, a name brand quality oil is sufficient, I used Castrol gear oil for about 20 years, for convenience, I use BMW gear oil in both bikes, makes it simpler for me !!!
I think that a quality oil changed on a regular schedule, will do the job .
You made the comment that the clutch 'failed' , what exactly were the symptoms ??