[crazyazn]

This is kinda useless but I'll put in what I observed a couple years back when I was comparing Shell vs. Chevron while I was tuning my car.

Chevron 94 gave me less knock than Shell 91, but the difference in timing I could have run wasn't significant. However, weather conditions may have been slightly different between the tanks and days I did tuning/datalogging. Range was about the same between the two. So back then, I decided it wasn't worth the premium for Chevron 94. Although the idea is popping back up because Shell increased the price for 91 and now there is only a 1 cent gap between it and Chevron 94...it used to be like 3 or 4 cents.