Good info. I was just speculating last night on this. Then this morning had some coffeeThe plugs are in the combustion chamber. The valves get dirty on the intake manifold side from heat/oil. I am very aware of this problem after owning several direct injected Audis. They eventually need walnut blasting. Other manufacturers including Toyota have added port injectors just to keep the valves clean.
https://www.consumerreports.org/cro/news/2015/02/pros-and-cons-of-direct-injection-engines/index.htm
"Some carmakers, including BMW and Kia, have issued technical service bulletins (TSBs) to their dealers recommending that drivers use only name-brand detergent gasolineāwithout ethanol additivesāand that they periodically add a fuel-system cleaner when they refuel. (A TSB is an alert that the automaker sends to dealers to warn about ongoing problems with individual models and how to fix them. It may allow dealers to make repairs at little or no cost to the customer as a goodwill gesture.)
Other automakers have devised an engineering fix that works while the car is operating. It involves modifying the engine to spray a small amount of fuel directly onto the valves to help keep them clean."
So, fortunately there doesn't seem to be a downside to using the additives. I live in a small town with less than 87 octane regular everywhere- so I was already planning to use a steady diet of at least mid-grade and occasional fill up with premium. Also, fortunately we have Conoco with top tier rated fuels and they also sell non-ethanol premium so I think I'm covered. It costs more for sure, but I drive very few miles daily, monthly, annual, and am retired and can afford the extra cost (consider it preventative maintenance) and can stick with my timely Chemtool routine and I'm good to go
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