Why is the throttle of direct injection engine dirty?

Turbocharger, air is, air inlet->; Engine air filter element->; Turbocharged air intake part->; Throttle valve->; Intermediate cooling->; Combustion chamber. The turbine is cooled by oil and water and lubricated by oil. The oil always overflows to the turbine inlet, and most of the oil is sucked into the engine by the oil-gas separator. However, the inlet temperature of the turbine is very high, and some oil evaporates and reaches the throttle valve with compressed air.