Topping off the tank often produces small toxic slicks in the water. Fillling the gas tank to the brim also forces out extra hydrocarbon vapors. A variety of these hydrocarbons found in gasoline help create ozone smog when they evaporate. Leaded gas is an environmental hazard. Airborne lead from motor exhaust causes liver, kidney, and brain damage in humans.
Scientists suspect that it is responsible for damaging crops as well.
Owners of boat motors built prior to 1976 often believe their boats must use leaded gas. The EPA says that this is a myth. It is the octane rating, not the lead, that is important. The only exceptions, they note, are when boats are carrying heavy loads or traveling at high speeds. Inspect your fuel lines regularly.