What is the problem of oil leakage under Ds5Ls?

Generally speaking, this is an installation problem, but it only needs the skills of the master in the 4S shop. The other is the valve cover that people often say, that is, what kind of black plastic cover, even if it can withstand high temperature, will be deformed because it is a plastic cover. I wonder how the turbine engines of other cars are. I only know that two cars of a friend I know, a Volkswagen and a SMART, have this phenomenon, so this is not a DS problem, but a basic phenomenon of turbo cars.