Pre 1975 engines were built for leaded gas. The heads did not have hard valve seats, and using unleaded will erode away the seats in a hurry. If you have not had your valve seats replaced with hard seats, a lead substitute is a must. Using a top oil like Marvel is fine for keeping carbon buildup on the stems. However we are addressing two totally different problems. Self serve came of age in the 70's, and a lot of people bought unleaded because it was 2cents higher than leaded regular they thought they were getting premium with tetraethyl lead. And it wasn't long before valve seats eroded to the point where the valves burned.