I had similar type problems with a diesel at 60K. I decided to remove the (plastic!) inlet manifold assembly and was amazed at the amount of carbon sludge/gunge and build up which I would not expect to be fully removed with an additive. I know mine was a dirty diesel, but whilst we all like to think squirting some magic stuff is a quick fix, I have rarely found it works in the long term. The problem is you don't really know the extent of the problem until you physically look and neither do you know if the expensive additive has been the long term solution.
I liken this to 'drain cleaning' . You buy some caustic drain cleaner and if you are lucky some of the blockage is removed. But later it comes back when it can be seen with a snake camera and only physical intervention cures the problem.