It is O.K. to be confused - when unleaded fuel was introduced in the '80s, even Nissan didn't know what to advise! Their poster would have originally placed 1977 and 1978 260Zs as unleaded compatible, 280ZXs as compatible with an ignition retard (no super unleaded available originally) and Z31 Turbo as requiring leaded. The problem was clouded by the octane requirement of these different engines - in reality, for the U.K. market, all these cars are unleaded compatible, but some need an octane boost offered by Super unleaded or Shell Optimax.