Today's modern oils are usually good for 20,000 miles with few exceptions - and that's not even talking about synthetics! The important piece of info is that you need to run your truck at operating temperature OFTEN to extend your oil this far.
If you're an occasional driver, the condensation that forms in the engine when it's cold will mix with the oil, and over time create acids and allow for rust and/or corrosion to form. The best course of action to prevent this is to change your oil at reasonable (4000-8000 miles) intervals.
What you can NOT skimp on is changing the filter! Regardless of how often you want to leave your oil in, your filter should be changed at the factory recommended interval of 7500 miles, or more often. The filter is made out of paper/cellulose, and as it gets old, it decays and begins to fall apart. If you get filter paper pieces stuck in your oil passages/bearings, you're going to have serious problems with engine longevity.
Me? I change my oil when it gets so black I can't see through it. *grin* About 5000-7500 miles for me, or every 4-6 months.
Greg