Hi Conman,
Intercoolers dont realy need servicing as such, but it will never hurt the cooler on engine to remove the cooler and give it a good hose out with some soapy water and a rinse. Both inside and out.
I have used a hose with a nozzle, You can use a high pressure washer but be careful not to bend over the fins as this will take some time to realign the fins.
Before cleaning check for a oily film on the out side of cooler as this will show up a crack in cooler. I have seen some small holes in coolers to let out what ever oil the turbocharger may by-pass. I dont know if Navara's have this type of tell tale hole.
Hope this helps.
VBSteve