Just a note to anyone who is having similar problems.
First i thought it was the petrol, but after two tanks this was clearly not the case.
If your S54's engine is running rough, surging, bucking and/or missing this may be the cause.
My vehicle only has around 17K km / 11K ml on it so not very old.
The fault codes were front Lambda Sensor Errors. Can't remember the exact code number.
These are very expensive little suckers. The two o2 sensors on the headers are the ones that I have read normally go bad. Each bank of three cylinders has an o2 sensor.
The faults cleared by themselves when I scanned the car again on my laptop after I replaced these parts.
So looks like that was the problem.
It also makes the job much much easier if you buy an o2 sensor removal tool. Worth the money just to get the job done.