For example, using 80W145: The oil at any normal operating temperature will ALWAYS be thicker than if the oil was any lighter grade, including 80W90.If you wish to think about this in a different way, imagine the oil is rated at 80W20000, & THINK about what that means. Link #1: From the BMW Factory School on the 5 speed gearbox and both early and late clutches.
Any oil will thin & thicken with temperature changes on 'its own chart curve'.
How this happens is a property of the base oils & additives.
In fairness, I will also note here that one experienced transmission rebuilder, Tom Cutter, my words here, believes synthetic gear oil in your Airhead is not good, leading to unspecified WEAR.
I also have NO objection to Castrol 80W90 GL5 oil in -synthetic; NOR do I have any objection to any of Spectro's GL5 rated gear oils, in appropriate SAE viscosities.
The biker community, like many other groups, has its own jargon, wit, and words of wisdom.