Are you loosing any coolant? Have the intake gaskets ever been changed and/or checked to make sure they are not leaking? Intake gaskets are common problems on these truck and if they start leaking, even a little bit, internally, they can make lifters noisy which could eventually lead to damage. That said, chaning your oil regularly, and making sure you have a good oil fitler, which is very important, especially with externally mounted oil filters, is important to avoid dirty internals.
If your not leaking coolant and believe its just a dirty engine, try gettting some engine oil supplement from a gm dealer and add a bottle in it with you oil(careful not to overfill of course) and see if that helps.