I've had a Focus ST since 2013. I now have 180000 km on it. The Ecoboost 2.0T did not ever have fuel dilution issues based on oil testing (I did three of them). From my experience, the biggest factors affecting performance is build up of intake valve deposits, and brake seize. By the time I got wind of this issue, I was already 80,000 km in (and did not want to muck around with valve cleaners in case I knock a chunk off and hit the turbine blades. I finally got a walnut blast done at 110000km. From the boroscope, there was significant build up on 1 of 4 valves. 3 of the 4 valves, while there was some deposits, it wasn't that bad. Since the blast, I've run CRC intake valve cleaner through the vacuum line just prior to every oil change (13,000 km OCI). The car seems to idle rough just before the OCI. But after the intake valve cleaner, seems to idle smoothly again. Like I mentioned, brake seize was also an issue on occasion. The manual does not mention brake servicing. But I'd do it at least after each winter.