Octane booster related.
Also ash deposits like that can and will eventually clog cats too. It's important to avoid octane boosters with MMT if you have cats.
He mentions it smells of grape. So it's most likely Manhattan oil, groovy grape. I find the Lucas fuel cleaner to work better and much cheaper, it's also an upper cylinder lube. 1 gallon is 20-30 bucks and treats roughly 625 gallons of gas.
I've been using e85 for the past 10 year. I use just Lucas fuel stabilizer, it's an upper cylinder lubricant and fuel cleaner all in one and their e85 stabilizer mixed together. 2.5 oz of each. Zero issues. I buy the cylinder lube in the gallon size and the e85 in their largest size offeres...
I live in surfside beach area in SC. Im from Stl Mo. I do it there when back in town visting. I know lots of people that fuel with their vehicle running, here and back at home