I'm normally really skeptical about this but had a friend service my car, and chucked in some fuel treatment, and its the first time I ever done it. I must say i'm getting a better fuel return even 3 tanks down the line by about 4/5 mpg. I do the same journey day in day out, not something I'd use all the time but pleased I did it. I thought I would just share my experience.
I was going to do an engine flush but got scared and didn't. Its using a little bit of oil now and thought the engine flush may clean the stuff keeping the oil sealed at bay.