Regarding mix oil and gasoline... Based on my purchase records, in recent years I've run at least 70 gallons of fuel through my Stihl chainsaws, trimmer/brush cutter, backpack blower, pole saw, and hedge clippers... All of it with non-ethanol 91 octane and Stihl Ultra HP mixed per directions at 50:1. The FS130 trimmer and HT131 pole saw have mix 4 strokes the others are all 2 strokes. I've looked at the pistons and they are all clean and show good lubrication.
I maintain multiple family owned properties and have done a lot of cutting on land trust (rail trails) and sportsmen's club properties. The only tool that ever needed the spark arrestor cleaned is the Hedge Trimmer. I recently checked the screens on all the chainsaws and they were clean... no build up at all. The FS130 valves have been adjusted once and were clean... not enough hours on the HT131 to even bother checking at this point. Spark plugs haven't been a problem either... it's a rare occasion to change a plug. The first Stihl chainsaw I bought was an MS271 and it was well used and maintained... I wore all the paint off the bar and the piston still looked perfectly fine with excellent compression. I upgraded my 50 CC class saw to an MS261 and sold off the MS271... I currently have MS170, MS461 and MS661saws too. Based on my experiences I don't understand why others have had bad experienced with the Ultra HP... it seems to work perfectly fine in Stihl equipment.
I've had pretty much the opposite experiences. I should really take more pictures, since this is very normal to see. Bg86 blower, very low hours. Stihl ultra 50 to 1 w/ e free 90 octane. (Same place I buy my gas from) would barley idle. Burned the screen out and all was well. Same issues at the township, even on moto mix, not limited to stihl equipment, they started phasing stihl out and going to echo. They eventually switched to the Echo power blend oil away from ultra and stopped having issues with choked up spark screens. Wish I could convince them to get red armor, but the power blend seems ro be pretty decent for the cost of it. Those guys will kill 20+ gallons of mix gas a week, between all 3 parks they maintain. If the road crew is tree trimming it's not unheard of to get 5-10 gallons through the pole saw, ms 180's and blowers in a day.
Now don't get me wrong, I work on enough friends/family equipment ti know people not knowing how to tune doesn't help anything, but carbon build up seems much worse with ultra then anything else, especially in weed wackers and blowers.
Edit: since we're on the oil subject I usually run 40 to 1, was running redmax/husqy xp switched to red armor and found both to be satisfactory, but that red armor sure coats a piston and seems to stick to everything internally. Been pretty impressed with it aside from the price going nuts.