Triumph Bonneville T140v - Part 11 - Gearbox Oil Change 

80/90 gear oil should be fine. My 2018 Honda Goldwing uses the same oil in the final drive. As a former Triumph Bonneville owner back in the late 70s I'm really enjoying your channel.

Really helpful video and extremely well explained. Same goes for your other videos. Thank you.

You can always anneal copper washers by heating until cherry red then rapidly cooling in water to soften them up before reuse.

Unless you have a serious oil leak, in which case you would be in danger of oiling your rear tyre, it shouldn't be necessary to check gearbox oil level while on a ride. You don't need to drain it to check the level. If you do want to check it periodically, all you need to do is take out the level plug (the small one), wait for it to stop dripping, then put a small quantity of oil into the filler plug until it starts to drip again. Plugs back in and it's done. I've always changed it every other engine oil change, as recommended, and use EP90, but EP80/90W is fine (they didn't have multigrade gear oils when these were made). In the 31 years I have owned my 78 T140V, the gearbox has never needed topping up between changes. I'm a retired mechanic, so always keep a stock of copper washers, and I would recommend replacing them, or at least annealing the old ones every time you take them out. Unit Triumphs don't need to leak oil if you do everything properly. Enjoy riding it.

Can the filler hole not be used to manufacture a dipstick that would be made to work bud? (hint 3D printed)