Volkswagen Repair: Octane to use, bosch platinum, vw jetta

Question
Hello Don:
I purchased a 2003 VW Jetta GLI (vr6, 24v) brand new, since i got it i've only used 93 octane fuel and this weekend a friend of mine said that it's stupid to use that octane if you can use 89 (cheaper) and still be ok. Is this true?

Answer
Carlos, thank you for your question.  I like these kinds of questions!  Usually, the higher the octane, the better your fuel economy.  But you can use 87 octane or 89 as long as when you are accelerating you do not hear the engine dieseling. Another way to tell is the sound will be the valves will chatter. But dieseling is a more accurate description.  Something that I always do is to run 1 tank of 93 for every 8 tanks of 87 or 89.  This will help keep the valves from developing carbon build up.  Also using a “hotter” plug like the Bosch Platinum or Bosch super plug in place of the stock plug will help keep down dieseling and carbon buildup.
Hope that helps