I used to be a proponent of various additives to diesel fuel and it was to address real or perceived failings in fuel quality. My old GM 6.5TD pickup really like Stanadyne Performance Formula. It actually got better mileage with it. But this was pre-ULSD. I noted a wide variance in fuel quality, and it was usually indicated by the Cetane number at the pump. Lower Cetane, poorer performance. My old Mercedes doesn't seem to care what I put in the tank, some additives make her run quieter but there is no change in MPG.
I have put a wide range of stuff in the fuel tanks of my diesels over the years. Vegetable oil, used and new motor oil, ATF, transformer oil, kerosene and Jet A. It all ran through them, blended with regular diesel just fine, in the short term. I have also ran all kinds of additives as well. Some will say that additives are not worth it. Maybe, but I have never suffered a fuel related problem with any of my diesels, not even a cold gel problem. Is it the additives? Maybe.
I now feel adding stuff to diesel fuel that was NOT intended to be burned in a diesel engine may be a bad idea long term. Motor oil, ATF, etc. Why do that when specific additives exist? They are cheap enough given the fuel use of most sailboat owners.
The only "treatment" I know works for sure is LubroMoly Diesel Purge. I used this on the Mercedes and it noticeably improved idle smoothness and reduced smoking. But that is a one-time treatment affair.
I do admit to adding Howe's Lubricator Diesel Treat to the tank in the fall when I top her up for winter. Supposed to help keep the fuel fresh, remove some water, improve lubricity, lower gel point, etc. Since I only burn $50-$60 in fuel per year the cost is minimal. Does it work? I can't say it doesn't.... I also like Marvel Mystery Oil but I tend to use that in the power equipment fuel or anything that gets used infrequently or for pickling engines for winter.
RT