For the turbo models you have a rock catcher, I mean oil cooler, sitting next to your intercooler. It takes its feed off the oil filter relief valve and dumps its contents out into the oil pan. After time the cooling fins get bent from rocks and no longer allow air to flow through. I took my cooler off to straighten those fins, clean it out good, and replace the two hoses leading to and from it. There was nothing that cause me to do this, I just decided "it was time." After pulling it all apart I didn't find the oil cooler to be clogged inside but its hard to tell since I didn't take quantitative measurements. At least now I have the happy feeling of a clean inside and a flowing outside. This took me all weekend because I was lazy and didn't want to straighten the fins out all at once. But if you do it should take about 3 hours with straightening the fins taking 95% of the time. If you don't want to do this all in one day get a long length of hose and just connect the two ends to "bypass" the oil cooler. I ran like this for 3 days. I think this also helps cleaned out those lines.
Instructions
With a pliers move the 4 hose clamps attached to the oil cooler hoses. In the picture you can only see three, but I bet you can figure out where the 4th one is :).
Now the oil cooler is solid Aluminum so it is easy to clean. I filled my with oil eater and flushed it a bunch of times until it came out clean. Then rinse it well with hot distilled water.