My dad's got a 99 Silverado with a 5.3 with 250k on the clock. Engine has never been touched, he hasn't changed his oil for a few thousand miles
That truck still has tons of power, doesn't burn a drop of oil, and still gets 20 mpg.
As mentioned above there are minor differences between the gen III, and somewhere around 04 or 05 they started putting bigger cams (like 5* more duration) in the truck motors.