I have a 2006 GMC Envoy Denali. It started using oil and the technician at my GM dealer said that there had been a problem with the lifters in that model, SO, they changed the lifters. I'm at time for my first oil change since changing the lifters and my car is 1/2 quart low on oil. Anything else I might look for? The car doesn't even have 50,000 miles on it.