WHAT DOES COSTCO GAS QUALITY MEAN?

I use Chevron 99% of the time. I haven't analyzed one other makes for comparison in my M5. Yrs in the past I owned a GM V6 which was very picky regarding the fuel I utilized, and I did monitor the performance variants with unique fuel makes.I also operate 76 when there is know Chevron useful, but I have been instructed that seventy six refines ther