When the hot burning gasses pass over the exhaust valves, the additives in the gasoline are supposed to cool the mating surfaces of the valves when they close. When there is not enough
cooling to the surfaces, it sort of melts away the valve seat surfaces. A couple of years ago my nephew asked me how the valve adjustment could get looser as the motor ran on his Harley
Davidson but the adjusters were still tight and hadn't been loosened at all. I told him that he was using unleaded gas and the seats were eroding away. He didn't believe me at first but he
had to buy new heads. Datsuns have bronze valve seats and they are usually not a problem with unleaded fuels.
THIS A IS DIFFERENT PROBLEM. One problem I had with my 1200, I bought a HAYNES REPAIR MANUAL a few years after I got the car. It listed valve lash as .010 cold for both intake and exhaust
valves. On a long trip, I burned a valve. I had it repaired and later, it burner another valve. After replacing valves five different times, I found a DATSUN TECH MAUNAL that listed the lash
as .010 for intakes and .012 for exhausts. I never burned another valve.