vronsky,

my 190e 1.8 litre car is u.k spec.

it has h7dco plugs which are replaced every 12,000 miles as per the maintenance booklet.

i suppose that with a colder heat range plug such as the the h9dco in a bigger engine car such as yours ,the car is lower revving at higher speeds than a 1.8 litre car,then the plugs on your car should last longer.

the spark plug gap for my car is 0.8mm as per the bosch specification.