I have an 81 horse II (I think). It does not shift like a car, by that I mean it wont snap into gear, but if I will move the tiller a little forward and back it just slides into position. Mine has the 140 wt. oil in the gear box and the gears look good when case top is removed.

It looks like a very simple gear box and should last a long time.

Mine has brass gears that run against the worm gear to drive the wheels, I think the tines are set up that way also.

I think if you remove the top cover and drain the lube you can get a pretty good idea what shape the gears are in, mine is 30 years old and looks great.

Larry

Changing Low and High speed always start form neural in low and high speed. First lower center lever to engage belt that move all income transmission gears (rotate). Then move low high lever to speed desired by transmission gears already turn an easy shift can be performed.

If lever still hard to move go to pivot on low high speed lever with 3/4 open wrench move nut counter clock wise half turn if this don't loosen level. Take all way off remove lever clean all parts rust is about freeze up pivot point. This pivot point is friction to hold lever in gear there no other detentes in transmission. You should be able to shift with little finger and friction tighten right hold in gear. Don't grease pivot and watch assy. and dis assy closely so parts be right place so friction work.

Center level that lifts engine lube with PB Blaster lube pins engine slides up and down on PB Blaster other solvent work as well. It take this about 3 times year make shifting easy for me.

This was discussed a few years back, about hard shifting. At that time, it was advised to use gear oil with the sulphur in it, as that made the brass gears slide easier. The artical said expressly NOT to use regular/ modern transmission gear lube. This pertained to the older tranny. Much newer ones may not need the sulphur oil.
Might i advise to go back several years in this forum, and look for the many words written about the older TB tillers.