I recently had my 07 Sierra serviced at the dealer where I bought it. I was told the dealers are now using only synthetic blend of oil for oil changes. The service tech could not tell me what the blend was other than being 5w-30 Dexos. Two (2) email later to the dealer and all I got back was a song and dance about the service they provide, etc. but no answer on what the blend of oil. I contacted the GMC Customer Assistnace Center and was unable to get an answer. I was told they would have to research it. All I want to know is what is the blend (%) of synthetic oil to regular oil. Can you help?