I have used both. Years a go I started only using the Red Max oil in my EBZ 8050 blowers as well as in my Red Max trimmer because thats what the warranty requires. That and at least 89 octane fuel, I always use 91+ just because. Last time I went to get oil the dealership was out of the Red Max so I used the Stihl and was impressed. I think the machines run a little free'er and I can vouch that the machines have less of an exhaust smell with the Stihl oil. My clothes reek of 2-stroke exhaust when I use the Red Max stuff!
Still, sometimes when I have extra race fuel left over from my go-kart I'll run it through my equipment. 110 octane leaded fuel with Motul oil baby!
In the end, I think the Red Max oil leaves more carbon build up as opposed to the Stihl. I have yet to try Amsoil but from what I have read I am interested in it for sure.