I think we need to be specific here. When leaded petrol was phased out there was much talk about valve seats receeding and the BMC A-series engine was thought to be one of the worst offenders. Much was written on the subject and the FHBVC tested a number of additives of which they gave their stamp of approval to about half-a-dozen, the which used about half-a-dozen different elements to provide protection from the damage. These are listed on the FHBVC website and are available widely from auto-accessory shops and elsewhere.
Many experts gave their opinions at the time, including John Rowland who was working for Silkolene Oils at the time as a chemist. John has a frogeye sprite amongst other long term motors, so is well versed in the vagaries of A-series engines - he produced a series of three articles which were widely circulated at the time and explained what happens, and how best to prevent it. His conclusion was that adding any of the additives would do your engine no harm; mixing the additives might well do your engine some harm; the additives cost money which might be better invested in eventually modifying your cylinder head IF your valve seats receeded; if you don't thrash the living daylights out of your engine the problem is unlikely to emerge (unless you are very unlucky).
Ive done about 30000 miles in TFM now on an engine that has had NOTHING done to the head and I use NO additives. There has been no sign of the valve-seats receeding and no sign of problems with the valves which are set with a gap of 15 thou. As the engine was out of a scrapper I have no idea how many miles it has done but I suspect LOTS. The Van is regularly used on long and short journeys at up to 65mph.
IF your valves receed, the first stage of reconditioning the head will include cutting out the existing seats and fitting hardened new ones, so no harm will have been done! Not only have I never heard of a valve being damaged in this situation (occasionally you hear of one having burnt out at the edge but that has nothing to do with receeding valves) but I can't remember ever having read of such a situation either.
So my advice (from experience) would be to ignore additives (unless you are going to thrash your engine) and enjoy your motoring. Check your valve gaps - BMC and Haynes recommended every 6000 miles in their service schedules - and only if they show considerable signs of closing do you need to worry about it - I do mine every 3000 miles as it doesn't take long and at the worst costs the price of a new gasket (£1.00 + vat). Having your own head converted to hardened valves and seats will cost you about the same as an exchange head (£150-200) but don't even THINK about it unless you KNOW it is necessary - I have known lots of people who have had the job done but only ONE who NEEDED it done!
Happy motoring!
Do you also run SAE 30 oil in your engine, I wonder??