Its all semantics designed to make them more money, they can charge more for a "blend" than conventional but the trick is that there is no exact standard of what exactly is synthetic, what is a blend and what is conventional... full synthetic can just be highly refined dino oil or it can be made from natural gas (dino molecules?) , also if you put 1% of whatever you arbitrarily define as synthetic into whatever you define as conventional then it is a blend. Straight 30 weight non detergent "conventional" is the easiest one to make but costs as much as synthetic because it is "special" for lawn mowers and model T ferds. It's all just a name game. Pick the cheapest one that comes in the weight you need and change it when it gets contaminated, they all get contaminated at the same rate.