What fuel should I use
On that car premium is recommended (I think, check owners manual) but I don't think it would be absolutely terrible for you to use regular.
Premium, the 320 uses 12 spark plugs, 87 octane does not burn hot enough and the engine will knock....
When I first purchased my 1999 ML320, I ran noting but premium. At 80,000 miles, I replaced 3 cats. Not long after that, I switched back to regular. At 180,000 miles, my mileage is the same, and I have yet to replace another cat. It makes no difference what so ever, except of course, your pain at the fuel pump.
My 1999 ML320 has 240,000 miles, with the last 115,000 run exclusively on 87 octane. With minimal maintenance it hasn't knocked at all & I'm getting 19+mpg in town, 21-22 mpg on the road.
Looking for a Used M-Class in your area?
CarGurus has 5,202 nationwide M-Class listings starting at $1,495.
Search Mercedes-Benz M-Class Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Mercedes-Benz M-Class Experts