I am being told by my dealer that my just purchased used 2000 DeVille DHS is losing oil because of carbon build up on the rings due to lack of use by the previous owner. There are no leaks or any reduction in cylinder compression. All other systems check out fine. The delaer plans to use some type of GM recommended cleaner that is supposed to reduce the carbon and therefore decrease the oil blow-by. Has anyone heard of this situation or am I being led down the dumb-dumb path?