I have two theories why some older vehicles may be better off with non-OEM plugs: Ethanol and waste spark.
The Ethanol gas thing is kind of obvious, it has a lower energy content and lower flame temperature than gasoline. That can be enough to push a pre-Ethanol vehicle to a different optimal heat range for the plugs.
While an older waste spark system might seem to be a natural fit for newer platinum plugs, sometimes the extra spark it creates a heat situation which causes these plugs to increase gap and burn out plugs very quickly. In these cases some folks have found old-school copper core plugs work better, and the replacement interval is not too different from burning out more expensive platinum plugs early.
The Ethanol gas thing is kind of obvious, it has a lower energy content and lower flame temperature than gasoline. That can be enough to push a pre-Ethanol vehicle to a different optimal heat range for the plugs.
While an older waste spark system might seem to be a natural fit for newer platinum plugs, sometimes the extra spark it creates a heat situation which causes these plugs to increase gap and burn out plugs very quickly. In these cases some folks have found old-school copper core plugs work better, and the replacement interval is not too different from burning out more expensive platinum plugs early.