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Thinking about switching to synthetic oil but I have a few questions. Should you stick with it once you start using it? I have about 11000 miles on my 06' CC, does the engine need to be flushed out before using synthetic? I talked to an Amsoil rep. and he told me that it did, he also said that he keeps his Amsoil in his truck for a year at a time. Synthetic or not, I don't think that is a very good idea. About how many miles should you put on synthetic before changing it? I do about 70%city and 30%hwy driving.