Older engines were not designed to run on unleaded fuel, especially those with cast iron cylinder heads. Such heads will generally have a valve scat that is simply a machined part of the ironed casting, so every time the hot exhaust valve closed it will try to micro-weld itself to the soft iron seat, eventually causing the seat to wear away. Once valve seat recession starts it rapidly progresses, compression and tappet clearances will reduce, the poor running, loss of power and increased fuel consumption being the results.

Converting and older design of engine to run on unleaded fuel is generally straight forward and only involves fitting exhaust valve seats of the correct hardness. Once fitted, most engines will run perfectly on modern unleaded or super unleaded fuel. Specifications of exhaust valve material must be carefully checked through most valves are already made of sufficiently tough materials, but if not, correct grade valves can usually be found or made.

With aluminum cylinder heads the original steel valve seat inserts are sometimes already hard enough to cope without adding lead, (only careful (laboratory) examination will determine exact material specification). The vehicle manufacturer, however, sometimes states that unleaded fuel must not be used. If this is the case the owner must approach the manufacturer and ask the question, Is my car unable to run on unleaded fuel -
a.) Because the valve scat material is too soft, or,
b.) Because the risk of pinking? (Bear in mind that super unleaded has a similar octane rating to 4 star.
If either pinking or valve seat material is the problem then try to pin the manufacturer down to another reason. Of all the older engine designs that will need converting for use with unleaded, only a tiny percentage will cause problems.