The weight of 600 litres of the oil in there is about 400Kg. I would doubt that you have a tank that could handle this - they are not designed to be lifted when loaded in this manner. A steel tank might handle it but it might buckle and, besides, depending on the age of the tank, parts of it may be weaker from corrosion. A plastic tank is a definite no-no.
There is a greater risk than the fire risk - the risk of environmental pollution if the tank breaks.