Because they would sell much more of it. It would no longer be considered the oil that the guy down the street sells out of his garage. Not saying thats a bad thing, but it would become "mainstream". Besides, if its not API, most people wont use it while under warranty. I know Amsoil has a guarantee, but why bother with the possible hassles. Again, think of the POV of the average consumer, not the BITOG consumer.