Yeah, in the North American market leaded fuel was phased out in increments over time from the late '70s through the very early '80s. By about 1982 there was well less than 10% of the lead in a gallon of 'gas' than was there in say, 1977. But other markets stuck with it for much longer. It's an issue with too 'soft' valve seats and the valves faces themselves. If the clearances are going away rapidly after a shim adjustment, or the adjustments are too frequent, then there's an issue. If not, no worries. The lead, as most probably know, offered a sort of cushion to the contact of the valve against the valve seat. With less lead there would be much more heat build up and therefore potentially more valve face and seat wear. Most Japanese bikes had 'Stellite' treated valves and hardened seats which resisted the heat of unleaded petrol as far back as the '70s, ironically.