How many miles on each shaft oil change??
thanks

Are you referring to the final drive (the part by the rear wheel)?
The manual says first one at 600 miles, then every 24,000 miles

There is no "shaft oil".

If you were asking about the final drive it's something that does not need replacing very often but you should check fluid level once a month. I change my gear oil when I'm replacing the rear tire with a spline lube.

A 1 quart bottle will give you several changes.
I change mine every so often for the heck of it.

final drive oil-- what weight? is SAE 80w90 good?

SAE 80/90 is fine - Just make sure it reads "HYPOID" and it never hurts to read the back to make sure it states for "Drive Shafts"!

You should also inspect your splines. To do this you'll have to remove the final drive. There is a step by step process here on the website by Fergy. Dry splines have been the death of more than one bike here.