Most direction for storage of a vehicle with a combustion engine has the following advice:
If you will be storing the vehicle for longer than 30 days, consider getting the oil changed. Used engine oil has contaminants that could damage the engine or lead to sludge buildup.
When an engine is used the explosive nature of combustion produces acidic residue which when the engine is not used will attack the soft metal surfaces in the engine. Modern oils are made for engines that are run on a daily basis with average mileage of 10,000 +annually, we all know that engines that are not run regularly have more duress and services should be more frequent. With oil being relatively cheap it is a good safeguard to change the oil which will lead to healthier longer lasting engine with what should be less sludge so more efficient. Of course you don't have to do that.
