I have wondered about this myself, and it got me thinking back to the time I worked in Subaru spare parts where i used to sell a lot of their Upper Engine Cleaner.
This stuff is way toxic and gives off some wicked fumes when the motor is started but many people swear by it.
We used to have mechanics from other dealerships come to buy it, which i thought was odd..... but that goes to show how well this stuff is regarded.
Search for Subaru Upper Engine Cleaner and you will find many posts about how good it is.
I have never used it myself but i think i will buy a can soon to clean out some 2 stroke mowers.
