I think you're mostly correct here. The majors can/do work with the additive suppliers on custom tweaks for what they're going after. But the vast majority of oils that meet similar specifications, take the Porsche spec for example, are going to perform very similarly.The extent of my motor oil knowledge is basically from what I have read and learned on this site. That said, it amazes me that there are any significant differences in oils of the same grade to even be concerned about. Castrol, Pennzoil, Mobil, Valvoline have all been around for over 100 years. After all that time, it would seem that they all know exactly what properties an oil has to have to provide protection, fuel economy, extreme temp performance, cleanliness, long engine life, etc. From what I gather, it seems like they all can, and do, buy a pre-mixed, if you will, additive pack from Afton or Infineum and add it to their base oil and have a finished product. I'm sure this is an over simplification but is it somewhat accurate? Does cost and cost alone drive the decision to have differences from their oil as compared to what a competitor is blending?
The reason why the top tier oils within a product line (ex - M1 EP/Castrol EP/Vavoline EP/PUP) cost more than their mid and lower tier is because they are in a sense boosted with higher levels of certain additives to exceed standardized tests and to be able to go longer drain intervals which require more AO's. In some sense you get what you pay for.
You'll see on most websites now: "20x more resistant to thermal breakdown" etc. ** Based on ASTM IIIH/IVA etc.