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Message: <blockquote data-quote="Schwepps" data-source="post: 360602" data-attributes="member: 3592">Haiyah, haven't I explained it before? Cheong hey la... I think it's good to ask though, then you won't go around believing the wrong info. It's just that I have an interest in chemistry, dunno why <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile    :)" loading="lazy" data-shortname=":)" />An analogy would be the best way to explain: mineral oils come from the ground and contain all sorts of things, even in the same oil field. G-I and G-II oils are refined by the fractional distillation method. No matter how good the process, not all the impurities can be removed. G-III oils are refined by hydro-cracking, which removes substantially more impurities. (It is G-III oils that Castrol USA wanted to call 'fully synthetic' and won the case against Mobil USA who fought against it)So if you think of making automotive oils as tennis ball manufacturing, crude oil will have all sorts of balls in it. G-I and G-II would have mostly tennis balls in the end product, but a lot of odd balls would remain: golf balls, baseballs and footballs - the impurities. G-III would have even fewer odd balls but there would still be some. G-IV, being synthesized in a plant from the molecules (polyalphaolefins) would have only tennis balls in the end product.</blockquote>