Why is there oil in my 2014 Nissan Navara's intercooler?

This is actually a pretty common problem and there seems to be two common causes. The first is leaking seals inside the turbocharger. The turbo needs oil to lubricate it, but if its seals are leaking they can allow oil into the inlet tract and, therefore, the intercooler.

The second cause is possibly even more common and involves the crankcase ventilation system allowing oil into the intake (and intercooler). A modern engine needs to consume its own crankcase fumes and burn them (rather than vent them into the atmosphere) as part of its anti-pollution systems. But in many cars (and modern turbo-diesels are the prime suspects) those fumes can contain some oil mist which, once enough of it has accumulated, will form the oil droplets you’re seeing in the intercooler.

While a faulty turbocharger will need to be replaced, if the problem is the work of the crankcase ventilation system, it still should be addressed before the oil mixes with soot from the exhaust gas recirculation system (another emissions-control device) and forms a horrible, back gunk that clogs the engine’s intake system. If hat happens, costly disassembly and cleaning is required.

Even if you’ve caught this in the early stages (before the gunk can build up) the oil can still cause problems with sensors inside the inlet system and could cause poor running and high fuel consumption. A good quality oil catch can is the preferred method of stopping this occurring as it 'catches' the oil mist before it gets to the inlet tract.