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Any oil will thin & thicken with temperature changes on 'its own chart curve'.
How this happens is a property of the base oils & additives.
I have no objection to it being used in the driveshaft (of those models using oil there).
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 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.
My present recommendation is that, if you want to, you should use a synthetic gear oil of good quality.
If you wish to think about this in a different way, imagine the oil is rated at 80W20000, & THINK about what that means.
Thus, at most any temperature you will be riding at, even after a full warm-up, the oil is thicker, a lot thicker.
A reasonable VI is assumed, and that is very likely what you will get on the standard oils, and I am not making specific recommendations, besides using Spectro's gear oils.
Better oils are now available, compared to when your bike was made, but 80W90 quality oils rated GL5 are still excellent, although a quality 75W90 may be slightly better, especially for those who ride (or start their rides) in colder temperatures.
While there can be other causes that a change to a thicker oil will not help, in this instance you can try the thicker oil, such as 85W140 or similar.