50:1 is 100ml per 5 gal while 40:1 is 125ml per 5gal, how do you get more buildup from using less oil, don't you mean you get more carbon deposit from 40:1 ?
If you buy a 6 pak of sthil oil they double the warranty on a new saw so I went with that and I mix 50:1
Nope, my observations reveal more buildup at 50:1. I find that if you're tuned rich at idle is when you get the extreme wet looking buildup that is what one would expect and is super common(especially in older saws), but with 40:1 and tuned right I have experienced little to no buildup. At 50:1, it seems like it burns hotter and fries the carbon onto the slug. The oil burns too, but not as hot as the gas so it all contributes to the cooling effect of the fuel. One of the mistakes I commonly see with saws is that guys will tune them like a dirtbike where the adage is that if you're not blowin blue smoke you're lean. Saws don't need the overcompensated cooling like a relatively big bore bike does so lots of saws end up super rich with clogged spark screens and wet pistons unnecessarily.