Two-stroke mix: 40:1 vs 50:1, which oil, and the one thing that ruins saws

Use the ratio on the machine (most modern Stihl/Echo/Husqvarna say 50:1); 40:1 is a safe choice for anything older or if you are unsure, as slightly rich on oil is harmless. Use a quality synthetic or semi-synthetic two-stroke oil rated JASO FD or ISO-L-EGD, not outboard (TC-W3) oil, which is designed for water-cooled engines running cool. Measure with a graduated bottle, not "a glug". Mix in the can, not the tank.

The saw killer is not the ratio, it is running straight gas once. Label the mixed can in giant letters. Mixed fuel goes stale in a month too; small batches.