I added a gallon of Ultra-low Sulfur Diesel (ULSD) at time thinking that my engine would get used to it more easily in small quantities. I expected that, based on what Id read about ULSD, it would dissolve a lot of old crud and gunk in my fuel tank and lines, and perhaps clog up my primary filter prematurely. By the end of the season, I assume I was then running on mostly all ULSD, and my engine never had a hiccup and the primary was in about the same condition it would be in with the older higher sulfur (5000 ppm vs 15 ppm) fuel.

I thought lubricity would be a problem, but I believe the refineries of ULSD are adding back whatever is needed for lubricity in order to avoid all the lawsuits that would follow if lubricity was really a problem. I do, however, add a bit of biocide, Pri-D, and cetane booster (all I can buy locally has a cetane rating of 40 and I think Yanmar specifies 45 to their engines) to each batch of fuel I put in the tank. Not sure I need these additives, but I dont think that they hurt in small quantities.