My partner and I noticed the same thing. I went with two tanks of the Costco fule and we noticed the power response was different. Totally drivable and it still delivered acceleration, but just not as much pep. Switched to Shell and BP and pep was back. I won't go to Costco anymore for fuel.
Originally Posted by Sanjaygb
I've been using Costco 91 gas in my 335i since August and felt the engine getting progressively worse over time. Switched over to Chevron 91 a few days ago... and I'm convinced that it's much superior to Costco 91. The engine is quieter when idling, and the engine's much smoother on the freeway. The reason I switched is because the car would shake quite a bit at startup. Read on here that gas could be the problem. The shaking has gone now too.
2013 X3 28i (prem, tech, cold1&2, conv, hud)