So here are a couple of things that I went through recently (over the past 2 years) with my 08 Tahoe Hybrid. The AFM (Active Fuel Management system which hydraulically disables 4 cylinders while cruising) is notorious for causing the lifter casing to crack then the lifters themselves sticking. This results in worn cam lobes. This results in your valves not fully opening, which causes misfires. I paid a local guy nearly $4k to do a full top overhaul, replacing the camshaft, lifters, head, gaskets, plugs and wires. I did that over a period of 3-4 months, $1,500 at a pop because he was incompetent.
Eventually it ran fine for nearly a year and a half. Then, I started losing a quart of oil every month. Different mechanics suggested that I just replace with a quart of Lucas. Then, last Thanksgiving, I was driving my family up to Tennessee. Oil pressure started dipping to around 20 PSI and started dropping slowly. I got a warning that oil pressure was low. I stopped and bought a couple quarts of oil. Kept going for another 200 miles. Oil pressure dropped again. Made it to Tennessee when the check engine light started flashing. Pulled off the highway in Chattanooga to get a code read from Autozone. But by then it was too late. Engine started knocking real bad. We limped to a dealership.
Long story short - Dealer took $6500 to replace the engine with a reman. 100,000 mile, 3 year warranty. Better than most used cars. Immediately, though, I've noticed that when running in V4 mode - sometimes - it vibrates just a little more than it does in V8 mode. That concerns me.
Less than a week after the engine swap, oil gauge started acting erratically. New engine, same bad oil sensor. Replaced that. No problems since (except the periodic V4 buzz).
Moral of the story - it could be a bad engine, caused by their faulty AFM system.