Originally Posted by slacktide_bitog
Honda actually did that indexing thing with the first-gen Insight. They list 4 different part numbers for their spark plugs in that application,one for each cylinder. However, the aftermarket only lists one par number. There are no problems with using the same one for all 4 cylinders
Is the GLC300 the same engine as the C300? The M274?
If so, NGK #90654 should work.
Rock Auto lists it for the C300 but not the GLC300, but they may be the same. The notes do mention it being specially indexed.
Normally you'd use the VIN and use EPC to look up the part number. The basic look up on the MB parts websites seem to say 270-159-07-00. You can get a knock version of the EPC/WIS on eBay for around $10. Anyway, once you search for that part number the MB dealers that discount online are cheaper than the aftermarket Mercedes dealers because the aftermarket ones tend to buy MB specific parts from the dealer and then have to mark up them afterwards. Some places that have it cheaper charge more for shipping so sometimes it pays to pay a few cents more and save some extra dollars on shipping but that's also a function of how far away they are from you. When you place the order, include the VIN so that they can confirm those are the right plugs for the car.
https://www.mynewbenzparts.com/oem-parts/mercedes-benz-spark-plug-2701590700