For repairs in the shop I fill them up with 89 octane, 50:1 w/ Stihl Ultra, and 1 oz of Startron fuel stabilizer.
If the customer uses it only a couple times a year I use 50 Fuel instead, it helps prevent comebacks.
My stock or muffler moded saws, I run 40:1 Klotz Original techniplate w/ 1oz Startron fuel treatment, 93 octane fuel.