Please check on what I'm about to write yourself since I'm not much of a mechanic.
Oil in the Intercooler Air Duct is not necessarily an indication of an oil leak in the Turbo. On my Yanmar there is a breather tube connected to the valve cover and to the air filter. The intent is that when pressure in the crankcase increases it is relieved by the breather tube. Pressure is caused by heat increasing the air pressure inside the crankcase, but mostly from "blow by", which are the gases that leak out of the combustion chamber.
These gasses contains some hydrocarbons, and instead of letting it drip under the engine the tube directs it to the air intake. If there is a lot of oil, that's not a good thing. If there is some evidence of oil streaking, or the intercooler is a bit wet with oil, that's normal. The amount of "blow by" is proportional to the condition of the piston rings, valve seats, etc. A worn out engine will have more "blow by" than a new one.
Of course, it could also be an oil leak in the turbo...