Let's identify a motor

That's not true, API sets a minimum standard and many oils have additive packages that far exceed those specs while others merely just meet it. There are also different bases they start with which makes a difference as well. I used to run Castrol in my GTO and once I got in the 1,000 - 1,200 miles my oil pressure was down 15-20 PSI. I made the switch to Kendall (same API rating and weight) and the oil pressure held rock solid until it was time to change again.