I've got a sneaking suspicion that the B20E might have had stellite exhaust valve heads, rather than the seats, due to the higher-than-usual temps as a result of the higher compression/power output/etc. Chances are that the seats are just the standard cast iron. Leaded petrol builds up a protective coating on the seats over years, which will protect them for a good while. To be on the safe side I've put a Flaslube kit on the 164E. Repco will do a kit for around $50 if you plead with/annoy them sufficiently. If you're not struck on the kit idea you can buy the stuff in a bottle and add an appropriate amount to the tank each time you fill up. Everyone I've spoken to swears by it.
Cheers

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Steve.