2006 MkV Running Fuel Rich
 

Would like a second opinion from someone. Have a 2006 MkV that has been running fuel rich since I bought it new. Dealer says everything checks out ok with the computer but did agree that it was running fuel rich (as evidence of heavy carbon buildup on exhaust tips). They consulted a factory engineer who recommended I quit running 93 octane gas and run 87octane. Dealer remarked that several folks have had this problem but there is no fix other than running lower octane fuel. Has anyone heard of other MkVs with the same issue and what if any explanation did they receive? I'm concerned all the carbon is going to clock the catalytic convertor and ruin the seals in the turbo. Thanks for any help you can offer