Basically nothing. Under hard acceleration though it’s not ideal. Cars designed for 91 usually run a bit more advanced with timing so a lower octane fuel would theoretically cause pinging. I doubt old man goes that hard though. Over the long term the engine might be a little dirtier with carbon buildup vs if they used 91 religiously. So yeah basically nothing. Oh and slightly poorer fuel economy.

Muriatic acid toilet bowl cleaner is an incredible metal stripper. It won’t hurt coatings bad or eat them up. Problem is it strips it so good you gotta primer it within a few hours or the moisture in the air will surface rust it quickly. If you don’t want to bother painting it, this method will strip it to metal and if you spray and rub in wd-40 or really any kind of oil it won’t rust at all.