I have a 34L that I picked up as a restorer that was very well used with 75k km. Pulled engine apart and 5th gear was badly pitted. No oil pump damage at all. Put in a new 3AJ pump regardless for good measure. I have 2 x 3TB's that had pitted 5th gears both after about 65k kms after very hard lives also . I rebuilt those engines to replace 5th gears and found no oil pump damage. I now run magnetic sump plugs now and they usually have a film of very fine metallic grey paste as expected on oil changes so they seem to do the job.

Gear pitting on big singles that do big kms is not just an XT600 issue. The same issue applies to DR650's and others. Because they have such low RPM torque it is very easy to ride them at low RPM which pounds the gears. I was guilty of this! After the rebuilds I changed riding style & change down a gear when required rather than chugging along in a high gear as I used to do. Replacing the cush drive rubbers I also consider important to replace regularly as these really take the edge of the shock loading into the transmission. I always notice how much smoother it feels after replacing old rubbers with new. They generally only last 10-15k kms before they are compressed and hard I find. Finally I now tend to run oil on the heavier end of the scale to theoretically give more protection to the transmission. Winter here in oz I run 20w50 and summer 10w60 ful syn. All good so far, have done 20k kms on one of the 3tb's. I will report back here in a few years to let you know how far they go before the next rebuild!!!

totally agree with you on all fronts.
i am actually considering mfg a small batch of 5th gear sets as they are not available anymore, got a good connection to do them for reasonable money. If i get around to make them, might go with a shorter ratio/less gap to 4th gear. At the moment 5th is essentially overdrive.

OK youre talking about early XT's? Yamaha still sell the 5th gear wheel 2JT-17251-10....just the 5th pinion gear is obsolete 5Y1-17151-01 . Im sure there would be a good market for that if someone made a batch?? I have an early XT engine requiring one! For my 34L restoration I ended up buying a transmission from an early TT600 from the US and using the 5th gearset from that. Is getting hard to find second hand good XT early transmissions with no pitting. The TT's did a lot less kms in general so the second hand transmissions available seem to be in better condition.

I ordered 2JT-17151-01 5th gear pinion from my yamaha dealer no problem. 4th gear pinion too.

Ive always bought the matching gear wheel where possible even though it might not be pitted as it will be partially worn to a different profile compared to new so may cause premature wear on the new gear and be noisy.

Not sure where you are but Ive ordered most of my parts direct from japan from https://japan.webike.net/ as many parts are generally half the cost of the dealer here in oz and I get parts a lot quicker. Dealer takes about 2 weeks to get parts from japan. ! week normally via webike. A 2JT-17151-01 from the dealer here was about $120 Au $55 au from webike

Yeah that's probably the best way to do it. Unfortunately the gears are almost 100 euro per piece. I'm not spending 350 euros on 4th/5th gears only. Especially since you don't know what will happen to e.g. then 3rd gear that wasn't replaced.