Love the video, good stuff.

But he did say, the cause of a failure after a long period without an oil change was the gear box being worn.
So if you follow that theory, with new fluid, the fluid is to spec. It's the gear box that its out of spec/worn.
I don't claim to be a gear box expert in any way, but I don't buy the theory at all. A normal corse of events might be, that fluid goes unchanged or the box just reaches the end of its life. Possibly the life ends early due to no fluid change but either way the box is wearing out. The driver senses a problem, first port of call, change the fluid. Might fix it if parts are sticking due to old fluid, might be a slight improvement followed by a complete failure because....the box is worn out.
Even to his own theory, its the wear that's the problem, not the new fluid.

That's a front wheel drive box, btw. Much less reliable due the lack of space and the need to offset the drive line due to fit it all between the front wheels.
Motorbike clutches look almost exactly the same as the ones shown, and suffer the same faults. Burnt out or warn plates, but also pitted marks/grooves on the clutch drum where the splines are under pressure. The plates dig in to the grooves and stick, holding the clutch plates apart slightly, then the clutch slips under load and that accelerates the wear rate.
