Good question. I think I would change the oil as it has been sitting and the oil has been absorbing moisture from the air for 15 years. For about a quart of oil and a little time, it would be a good investment.
Be sure to use the specified viscosity, such as SAE 30 and if so, do not substitute SAE 10W-30. Also, be sure to use fresh gasoline and treat the fuel with a stabilizer so it will remain fresh longer. The shelf life of gasoline is only about 90 days, so I suggest using what gas you have in a gas can and pouring it into your vehicle. Then purchase fresh gas for the generator regularly.