Searched archives about octane ratings for 2.1 wbx engines, but not the answer to this specific question:

  Two days ago our well-running 1988 Vanagon GL Camper, 2.1 original engine, was filled with the higher-octane gas at Sam's Club by a well-meaning person. I believe the choices there are 87 or 91, and the choice made was 91. Also should mention we are at an altitude of 7000 feet.  Maybe I'm imagining things but the engine is no longer purring as smoothly.

  So. Is it ok to run out the full tank with the higher octane? Although I guess there aren't many alternatives to doing so....
  Thank you,
  Patricia

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