You're not getting any of the alleged benefits of synthetic oil (extended oil changes, and reduced wear) since you put so few miles on the engine. The biggest issue would be water condendsed in the engine that is not getting evaporated out regularly and maybe some carbon pollution from running around town and not getting enough sustained highway time to burn it out and clean up the carbon. I would just run conventional oil and change it once or twice a year. IMO anything fancy is just a waste in a truck motor that gets used so little.
http://www.supermotors.net/vehicles/registry/9026
