"I need to store my bike for a few months and the shop said I should add fuel preservative so my gas doesn't go bad."
It's always a good idea to add fuel stabilizer/conditioner before parking anything for an extended stay. In fact, more than just adding it to the tank, it's better to include it before your last ride so the stabilizer spreads through the entire system.
Fuel is made of many different organic materials and over time these compounds can change as they react with elements like oxygen and moisture. When the fuel changes it can take on undesirable forms like varnish or a gummy residue-both of which are horrible for your bike.
By adding a fuel preservative you can extend the life of the fuel to over 18 months without contamination compared to untreated gas, which is typically spoiled in a few months.
According to Dave Willis of Spectro oils: "Our Spectro FC Premium inhibits corrosion in a fuel tank and system by absorbing/encapsulating water that accumulates naturally and attacks metal surfaces and speeds up the deterioration of gaskets and O-rings. The oil we add to our product not only inhibits corrosion and provides lubricity, but also, as the fuel inevitably evaporates, the coating of oil prevents a layer of crust/gum from forming, allowing for easier starting after storage."
As an alternative, some people prefer to drain their bikes of fuel before storage, but the gas residue can still turn into varnish or sludge, and steel gas tanks can even rust on the inside if left empty for too long.
So when in doubt, fill it up and add the special sauce.