A common cause of failure in a combustion engine is lack of lubrication causing wear. This wear happens where oil is hard to reach. The head of the engine is one of these locations. There are a few things engine manufacturers do to help this lubrication. One of them is to put the top of the head under vacuum. This negative pressure helps bring the oil to the valves & keep it there. It also prevent positive pressure being built up in the head & therefore pushing oil out of the rocker cover gasket. The down side is that this oil is very hot & hence gives off a vapor, much like deep frying something on the stove or in a deep fryer - you notice the vapor that collects either on the wall near the fry pan or on the underside of the deep fryer lid.
  The way that engine manufacturers achieve this negative pressure in the head is by connecting a hose from just before the turbo to the top of the rocker cover. The line from air box to turbo is under a fair bit of vacuum so thats why they take advantage of that location.
 The problem is that the hose from rocker cover to intake tube is full of oil vapor. This vapor then runs thru the turbo, into the intercooler & into the intake. It is here in the intake that the oil vapor hits the hot EGR gas & sticks together into a cake. The hot oil itself is not much of a problem, it is combustible. If you clean your intercooler out you will find there is usually a little oil in there & it should look pretty clean (not black) as long as the oil change intervals have been followed (every 5k). This is from that rocker cover vacuum line. 
The catch can has a media in it which is fine enough to collect most of that oil vapor  mist where it all collects together as it cools & forms a true liquid, this liquid runs down to the bottom of the can where you can usually drain it. Like the lid of a teapot when boiling - the vapor forms a liquid when all the molecules cool & cling together. Like distilling whiskey (not that i condone such behavior, but i sure do support it). So once the media in the catch can is saturated the media must be changed, usually not for 50,000 to 70,000 kms (check with your can manufacturer). 

 

Properly engineered Catch cans work great at reducing the oil in the intake, no doubt about it, but they do nothing for reducing the hard carbon particulates formed from incomplete combustion . And catch cans are usually installed without affecting the warranty - but check with your isuzu dealer first..

 

If you are going to get a Catch Can make sure it is a reputable brand. It is no use wasting money on a cheap can that does not catch vapour.