It sure is a petrol thing. Diesel could be deemed a light lubricant. Keeping a petrol tank full or close to it minimises the volume of air for the water to condense from notwithstanding the modern vehicle has a plastic tank and operates in a sealed environment.Dirty tanks must be a petrol thing. When I pulled the sender out of my old 504 diesel after 25 years the bottom of the tank was as clean as new.