The reason I run 40:1 is three fold. It is a compromise between fuel economy and engine protection, like I said I run as lean as I feel safe at lower elevations, this is also why I run C12. I do a lot of long hill climbs and ride with fast AA enduro guys, so needless to say I still work my engines pretty hard. Second 16 oz to 5 gallons is just too easy, as many oils come in .5 liter and 16 oz bottles ( I know .5 liters will not make 40:1, but it is close). Third, it has become the agreed to standard with my riding buddies, so we can share fuel on long rides without it effecting jetting. Though I do have a friend that brings extra oil to adjust his jetting if his bike starts to run ratty at higher elevations.
One of my friends uses Castrol A747 in his CR500AF. On the last ride with with him I made a down hill death pass through the brush to get away from the sickening stench. It is good oil, it is the most widely used oil in Grand Prix Bikes in Europe, but it stinks to heaven and goes for $21.95 a liter. I have had no luck trying to get him to change oil. Personally I find the Amsoil to have a slight castor oil aroma. I think it is common courtesy to run a good smelling oil for the sake of those that have to ride behind you on tight trails. From time to time you can find me putting my nose to the silencer of my bike as I walk through the garage; mmmm, smells like freedom. :confused: