My R100RT BM suffers from valve seat recession even after fitting 'unleaded conversion' valve seats. (Additives like Valvemaster Plus make no difference). Didn't have any problems at all when it was running 4-star. My theory is that these days no-one apart from me actually rides one regularly for any length of time above 70mph. (Not in this country obviously
When unleaded first came in I was riding a VH with the 52-53 cylinder head. I tried it with unleaded as an experiment and had massive valve seat recession. However most people claim no problem with unleaded and iron cylinder heads (although it might depend on use - the same VH, now fitted with an alloy head, currently only does a few hundred miles a year and even if it was suffering valve seat recession I doubt if I'd notice it.)
The British car industry experimented with different grades of cast iron in engines. I think later A-sries Leyland engines use much a much softer grade than earlier ones and consequently later cylinder heads suffer more from valve seat recession with unleaded. I guess it's possible that Ariels followed the same route and the last of the cast-iron VH (and NH) heads might be made of a softer grade material than earlier ones, which might explain why mine wouldn't run on unleaded.
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)