I've never changed mine in 7 years and was told that the most important thing was to check the oil level at the beginning of each season. Don't fill outside and allow any dirt to get in. Clean the outside of the bottle before you use it as dist/dirt could fall in as you add oil.
I don't recall the dealer telling me to change it. It isn't like motor oil that has to be changed due to the heat, breaking down the oil etc.