In Australia, Shell fuel is an absolute no-no in aircooled machines - particularly the higher octane "Optimax". This stuff is truly dreadful, fouling plugs by coating the porcelain with something that obviously conducts, so the spark never jumps the gap to to the elecrtode. I had terrible running on my T160 Trident, going through several sets of plugs until it dawned on me that the problem was the fuel. My brother and a couple of mates went through the same ordeal on their historic race bikes - using 10 plugs between them in one afternoon when using Optimax. They changed to BP the next day with no problems at all. Shell are evidently well aware of the problem, but how much of the market is using aircooled machines these days? Very few obviously, so to them it isn't an issue.
I should say that I have had no problems with other brands of fuel, BP, Caltex or Mobil being the main players here. As to fuel with ethanol in it, well that has been an issue - but from talking to a couple of fuel chemists it seems the problem is with unscrupulous servo owners doctoring the fuel with toluene - a well known scam here to avoid paying duty on the fuel.
Like Eddie, I also started using an additive with the fuel when leaded fuel disappeared, but have long since stopped doing so with no problems as far as valve seats go. I sometimes throw in a splash of 2 stroke oil as an upper cylinder lubricant when I remember to do so. I've been told this helps with engines generally, but in all honesty I think with the small mileages we do on most of our machines it's probably incosequencial. Having said that, a couple of the veteran machines have done a lot of miles over the years, particularly the 3 1/2hp outfit, which works pretty hard. I think I would have seen signs of valve seat recession in this machine if it was going to happen. So folks - don't be taken in by the doomsayers!
Cheers, Graeme