These pictures show what can eventually happen to 1108cc exhaust valves!
It's one of the valves from my F6, which I was lucky enough to find twenty years ago when it had only done 30,000 miles. I did another 30,000 miles on leaded, and when this became unavailable used an additive rather half-heartedly for a while. I then ran it for another 120,000 miles on unleaded (95, no additive) without any performance/consumption problems.
In 2009 we went on a trip to Scotland, and after 350 motorway miles at high (for an F6..) speed, it began to miss quite badly. I thought at first it was a timing problem, or that the points had closed up, but when we got home (even on 3 and a bit cylinders it still did 45-50 to the gallon) I took the head off to investigate.....
When I had my engine professionally rebuilt they found an awful lot of black soot inside but the engine was fairly good wear wise. I had been using additive religiously and the higher octane (performance) petrol. Its now got harder valve seats etc to make it run ok on normal petrol.
There now Ive probably stirred up the Additive No Additive debate again