No, you should not use unleaded gasoline in a 1961 Cadillac without modifications. Cars from that era were designed for leaded gasoline. The lead in the gasoline helped lubricate the valve seats. Using unleaded fuel can lead to premature wear and damage to the valve seats, requiring expensive repairs.
To safely use unleaded fuel in a 1961 Cadillac, you need to have the valve seats hardened or replaced. This is a relatively common modification for classic car owners.
