cikeijr, Blow BY is caused by either worn piston rings or bad cylinder walls. Most common are the rings. The older and more worn the piston rings and cylinder walls of an engines gets, the more likely it will allow greater Blow By. By definition it is "leakage of the air-fuel mixture or of combustion gases between a piston and the cylinder wall into the crankcase of an automobile. " from here. This will of course increase the pressure on the inside of your crankcase and thus push the oil out through the dipstick and other passageways. To test for Blow-By, service centers have a special tool that connects to the engine as well as a magnehelic gauge. Check on here to take a look at some of the tools of the trade. Download the drawing and spec sheets. If you want to check for yourself, I would recommend a leak down test to see which, if any particular cylinder is giving you trouble. Hope this helps, good luck.