what's the requirement, or minimum requirement, for an oil to be labeled as synthetic these days? Let's say I buy brand x oil, and the bottle says either synthetic, full synthetic, or 100% synthetic. At worst, what do I know I am getting? Does the classification of synthetic refer only to base oil types (1..5)? or do additives, or something else, have any part in it? what about "synthetic blends"? what is the minimum requirement for an oil to meet the syn blend labelling?