I disagree, grey smoke is not oil burning.

Bright Blue smoke is a sign of burning oil, an easy way to check if oil is entering the combustion chamberi s to check the spark plugs, see if they are fouled.

My opinion on grey smoke is an over rich mixture.

The next statement is purely speculation however, My 190e had a faulty EHA valve, as a result the car was running overly rich and grey smoke was evident from the tail pipe over 3,500 revs under load.

When did the car have a service, IE air filter, plugs, mixture, etc. Because a clogged air filter will cause this, along with an over enriched mixture.