In your motor, the piston is moving up and down in its cylinder, and the rings on the piston are creating a seal so that when you ignite an air-fuel mixture, that reaction is converted into power. As good a seal as the rings make, they will never be perfect. What this means is that some of that expanding gas will get past the rings, and then it will be present in the crank-case.
First, keep in mind that we are talking about a very small percentage of pressure here (as long as your engine is in good running order), but that pressure still has to go somewhere. For this reason, your car has a Positive Crank-Case Ventilation (or PCV) to release this pressure, and it will rout it back into your intake system.
There are two reasons that this is sent back into the intake, rather than being sent out to atmosphere. The first is that Automobile manufacturers aren’t allowed to emit anything directly to the atmosphere for environmental reasons. The second is that it can actually be advantageous for there to be a slight vacuum present in the crank-case because then there is less on the piston’s down-stroke.
However, because this air comes from the crank-case where there is a lot of oil present, this air usually has some oil vapor present in it. This oil is also referred to as blow-by, and in moderation, it is completely normal.
In a Subaru WRX for instance, it is completely normal for there to be a thin film of oil present on the intake plumbing, and for there to be a drop or two of oil pooled between the intercooler and throttle-body. That being said, if you find a more substantial amount of oil present, it could be indicative of a larger problem, so it would be a good idea to find out the cause.
If you are running more boost than stock, there is a chance that you will send more oil through your PCV system. This will especially be the case under hard driving. This is partly due to the fact since you are pushing more air into your motors cylinder, there is more air (under higher pressure) that is getting past your rings. This is one reason that it is a good idea to keep an eye on your oil level in between track sessions, and in between oil changes for instance.
One way to help minimize oil getting into your intake is to install an Oil Catch Can. The idea here is that you will cause a drop in pressure, and cause the air coming out of your PCV system to make a sharp turn before entering your intake, and this will cause most of the oil present to get trapped in the Catch Can. Most Catch Can’s also have a level indicator, so you can see how much oil you have accumulated, which can be a helpful tool to remind you to top-up your oil.
If you install a Catch Can, and you see that you have an excessive amount of Blow-By, you may want to have a mechanic inspect you car, and do either a Compression or Leak-Down test to make sure that your rings are still creating a sufficient seal with your cylinder-wall.