Are you in Australia? . If the answer is yes,then i suggest you change servo's because it's very probable that you have been sold 89-94 octane as 98. It's a common thing in Australia because there is no way to check the fuel we are sold.
I just spent a couple of weeks travelling around OZ in my 6.3 and more than once I filled with alleged premium unleaded only to find the car was pinging like crazy.
There is only one cure. Add 1 liter of ATF to every 100 liters of fuel.
then,drive the wheels off the car!.The ATF not only lessens pinging,but cleans out the MFI and the injectors.
Pinging can be caused by several other things besides fualty ADVANCE /RETARD MECHANISMS.

First thing to do is fit the coldest plugs you can find .
Next ,check the thermostat and make sure it's opening at the temp stamped into it.
Flush the block and radiator to make 100% sure the cooling system is working properly. With the engine running and up to temp,the water should be seen moving across the radiator
Change the aircleaner element to make sure the cool air is getting to the engine.You'll notice Mercedes was decades ahead of the game with the aircleaner snorkle in front of the radiator instead of sucking in hot underhood air.
Check the crankcase breather pipe is clear.
make sure the fuel pressure is 14PSI and no more. Over pressure causes all sorts of drama with a MFI system.
Make sure the plug leads and the plug ends are Mercedes type,not elcheapo silicon colored leads which have far too much resistance. if you have the original type ,run a multimeter over them to make sure the Ohms are not too high.
If you know anyone with a CO tester ,check the mixture underload at 60 kmh. If it's too lean you will get pinging because everything will be getting damn hot but not showing up as a hot engine on the gauge.