Water can also leak into a tank from ground water when the oil level is low if the tank is damaged. Tanks may also be damaged by being dropped or pushed into the excavated hole rather than being carefully lowered by a rope. We do not consider a minor drip onto the boiler room floor at a bad fitting a reportable leak for these purposes. What to tell your client A buried fuel tank is installed. Such components are not inspected during a home or building inspection unless specific prior test arrangements have been made for advice by an appropriate expert.
Some general advice is below.
Water corrodes the tank and leads to leaks. While we've found them lasting longer, a common life expectancy of buried oil tanks is 10-15 years. We do not have the same data for gas tanks. Life may be similar. If the tank is to remain in use, ask your fuel supplier about using an additive or other methods to help remove water.