I used to buy it, because it was there and I had read all the scary stuff about valve seat recession (despite the more positive stuff about 'lead memory'). And boy, there's lots of it, from oil companies, chemists, you name it. However, in common with many of my friends, I don't bother any more.
I have not had any valve trouble on any vehicle. I think tin pellets are snake oil The liquid additives may be better but I don't know and can't be bothered to carry the stuff around with me.
When 4 star fuel, leaded, was withdrawn here in France about 4 years ago, the refiners suggested using their additives with the 98 Octane (89RON I think) (best stuff). I use only 95 (85RON?) in everything and in a fair few miles have had no problem.
Iron heads: the A10 has done a good 10,000 miles without additives and the valve clearances haven't altered. I don't thrash it, but run it at around 60mph. B31 has done fewer miles (haven't had it long) but no problems so far, in about 2000 miles this summer/autumn. On my alloy-head AMC twin, I have almost never had to adjust the valve clearances. I am running that engine with the 3 of the same valves and all of the same guides and seats it had in 1976 when I got it. It has probably averaged 1500 to 2000 miles a year (having gone nowhere for most of the '90s). It's been doing a good 4000 miles a year recently. The lack of lead has made absolutely no difference.
'They' do say that if one keeps the revs to about 3000 rpm, seat recession won't happen anyway. I maybe stay around that mark mostly on the BSAs, but not on the AMC 500 as it has to be revved to get anywhere.
So I'd say that the worries are over-rated. At least for cooking engines. Has anyone had the opposite experience, attributable with reasonable certainty to nothing other than unleaded fuel I wonder? If so, I can start worrying again.
