I recommended it to him but he says that it makes the engine cough and splutter - could this be possible?
I don't mean the lead in the fuel, Mercedes have been able to run on unleaded fuel since the early 80's. I mean the octane rating. The car should run smoother on a 98 octane than a 94 octane.

What your probably find is happening is because of the higher octane rating,the super fuel needs more air to burn off the fuel,Which is by rights causing the car to overfuel which would cause it to splutter in some cases,But dont stand near the back of the car and spark up a ciggie!