The biggest advancement with modern oil has to do with the roller cam. Flat tappet cams need ZDDP (Zinc Dialkyl-Dithio-Phosphate) which essentially fights detergents in oils. Zinc causes the oil to break down quicker and is essentially used in the motor as a lubricant in metal to metal parts like lifters. Roller cams don't need as much zinc, so higher detergent levels can be used. Synthetic molecules have less contaminants because there are no dinosaur bits in it and have higher molecular shear charicteristics than conventional oils, especially when compared over time.