sooo.... some pple use synthetic on their GSRs, and some don't. Do you really notice a diff in how the car runs using the synthetic, because from what mechanics have told me, you're still supposed to keep with the manufacturer's recommended service intervals regardless of the oil you use due to the fact that it isn't the oil that wears out that causes you to have to replace it, but the debris that are suspended in the oil that need to be cleaned out of your engine.