It is my understanding that the valve seat recession problem applies particularly to the exhaust valves. Valve inserts in the exhaust ports on L-series heads are, to the best of my knowledge, steel, not bronze which would be too soft to cope with the exhaust valve heat and hammering. Inlet valve seats may be sintered iron or bronze, but steel would be recommended for higher power engines(?)
I'm not convinced that the L-series head will suffer from valve seat recession (VSR) when using unleaded petrol and there is seemingly a lack of information about materials used in valve and seat manufacture on earlier engines. The problem of octane requirement remains. Later 260Zs were officially listed by Nissan as suitable for use with premium unleaded (all that was available initially), but 240Zs and early 260Zs were not included on their lists - I would presume for octane reasons rather than VSR problems. In the USA, all L-series engines from approx. 1974 on would have to be unleaded compatible as no leaded was available from this date. This leaves just 240Zs and early 260Zs with a question mark. There is a part number change for exhaust inserts effective from 04/76 in 260Z heads, but what this relates to I am unsure. The valve does not change at this time.......
The safest option would be to use super unleaded or Shell Optimax and at least occasional use of an approved additive (like Millers VSP) or tankfuls of the leaded fuel still available from selected sites.
I have no long term test data relating to the Z engine, but the main problem of VSR seems to affect mainly cast iron heads or heads with sintered iron inserts. The expense of a "conversion" for L-series heads is probably not warranted!
Who out there has had an "unleaded" conversion and what was done?? Any problem side effects, e.g. new valve seats dropping out?