It is possible that there is not that much difference in viscosity between
5W-40 or 0W-40 .
Rotella synthetic versus Mobil 1?
Additive package in Rotella is more robust, but will it be better for a gasoline engine?
It depends.
High performance engine driven hard, maybe not.
Regular commuter driving with lots of cold starts, Rotella probably better.
Garage Queen, often not driven for weeks at a time or on weekends, Rotella is probably better.
Car with expensive catalytic converter, kept for a long time, and used in a state with stringent emissions testing, Rotella might have too much phophorous.
Base stock might be better on Mobil 1 than on Rotella. I don't think that Mobil
1 has switched to Group II base stock.
The European and Japanese rating systems might be a guide in determing which oil is better...and meeting a manufacturer's specific rating or test is worth considering.
The 'commercial' oil might not be the best, since the better oils are really a balancing act between many engineering requirements and the govenrmental and corporate interests.
I don't think it comes down to 'Supermarket vs Commercial oils' , but rather meeting the manufacturers specs...or knowing the pros and cons of varying from or trying to exceed them