Stihl didn't have an Ultra when I left the company in 2005 , all there was at that time was the regular oil made to our specs by Castrol and it was a clean burning oil .
Why Stihl would use a lesser oil in the Ultra does not make sense at all . All I can think of is they were trying to produce a cleaner burning oil for the 4 mix engines that were having a lot of issues from carbon but by the sound of it ultra isn't even near improving that and it's been on the market for several years so I'm sure they must be aware it's not working well and I'm sure they have had lots of complaints .
Strange for a company such as Stihl to let something like that slide and not improve it .