J-150 is right. The 3,000 mile myth is still going strong. In addtion, going with a full syn (ie, Mobil1) your going to be fine running it to 6,000 miles between changes under 'average' driving conditions. Tests have shown M1 is good for well over 10,000 miles.

I changed my '99 Expy every 5-6,000 depending on my schedule and motivation. That motor was just as strong as day one @ over 100,000 miles. My 'bro's 92 suburban was good for nearly 200,000 miles before he scrapped it (and he didn't scrap it 'cuz it wasn't running, it had plenty of life left!).

Use good oil & filters and change every 5k give or take. Your truck will have a long, happy life.