Stromboli,
High viscosity gear oils don't normally attract moisture as quickly as fuels if properly sealed. They do however degrade as any hydrocarbon will over time. If you check recommended oil changes on engines the almost always include a time period such as "one year" along with the mileage.
I used to change the gear oil (Yamalube) in my FJR rear end every other oil and filter change (3-4,000 miles). However if the bike had sat for a time I would change it anyway. Some might think that I was wasteful but oils are the lifeline of any mechanical system. Oil is not cheap these days but compared to a rebuild on any engine or drivetrain it's not expensive at all.
