I used to think the same thing untill a few weeks back when i had this discussion in a previous thread too, i compared the specs of XQ 10W30 (fully synthetic) and Ultra 7 20W50 (mineral) and found that they both behaved in very similar ways regardless of the SAE rating (i'm looking into why still).
as for the API ratings, it has been found that sometimes putting an SL or SM in a car which requires an SG may not be a good idea (however this mostly applies to porsches with catalytic converters).
also the APi rating has little to do with the total additives in the oil, it's a rating to help determin the behaviour of the oil (along with some protection features) but every company adds the additives in different ratios and therefore the API rating is just a guide rather than a rule. but you are right that an API SL or SM will for sure have better additives than a API SF (for example).
