I bought my B6 A4 1.8T 5MT with 99517 miles on it and had the oil changed with Castrol 5w40 and Fram filter.
The car always felt like it was struggling with every shift (up or down) and doesn't want to accelerate until 2,500 rpm. Replacing AutoLite Double Platinum spark plugs and air filter didn't help either.

At 102020 miles (2,500 miles since Castrol oil change) I put in 1 Pt of SeaFoam in the crank (5Qt of oil in there) and drove like 3 miles and changed oil after the car cooled down. I could have driven a bit more but I felt it was enough distance and time for SeaFoam to work on cleaning my engine up.

Sure enough, now I can get the car to accelerate from 2,000 rpm and the boost deliver is sooner and smoother than it used to be.
To be fair the new Royal Purple 5w-30 oil and Mobil 1 filter may play a role in the increased power delivery but I don't think they did it all by themselves since the car has been lagging on power delivery even after the oil change before.

 

I'm considering doing this to a 2002 B6 A4 soon to remedy similar issues with late power delivery. Is your A4 still running smoothly after you did this seafoam treatment in your oil?

Thanks in advance.