I am having maintenance done on my Yanmars now. The mechanics are replacing impellers, filters, hoses, oiil, descaling, etc. Nothing catastrophic. I asked the same question about additives. The head mechanic with 15 years experience said never add fuel additives. You end up loosening grime and growth, which will clog your filters. He recommended doing a fuel polishing, which is a flush and cleaning of your fuel tanks. The mechanic said he could tell if I needed a fuel polishing by looking at the fuel filters. I didn't need one.