Casrol Valvemaster plus
has an octane booster so brings fuel back up to levels that used to be and has a lead substitute, doesn't cost a lot, use at 1 ml per litre.
Lots of differing opinion here and everywhere but I've used it four fifteen years now and not had any trouble , never had a stale fuel problem or valve recession, some will say I might not have had any bother anyway but it's down to personal choice