I run Chevron Delo 400LE 15W-40 with the Fram Extended Guard filter.
***This blows a lot of people's minds somehow: If your oil is clean after 3k, or 5k, or whatever, it is NOT doing its job. Dirty oil after a short period is a GOOD thing. That means that the oil you used did a lot of cleaning. Think about washing your truck; when it is dirty, the water that comes off is dirty. If the water comes off clear, you aren't getting anything off your truck. I highly doubt any GMT400 has a beautifully clean internal engine (if its original).