If your vehicle is showing visible oil leaks or thick oily residue on the plastic air intake pipes, here are the most likely causes and the parts you'll need to fix each one.
The Positive Crankcase Ventilation (PCV) system routes engine gases back into the intake to be burned. A stuck PCV valve or clogged oil separator allows liquid engine oil to be sucked directly into the air intake pipes.
If the breather hose connecting the engine valve cover to the intake pipe is cracked or was left loose after an air filter change, oil vapor will seep out and coat the outside of the plastic intake pipes.
Adding too much engine oil during a recent service increases crankcase pressure and causes excessive oil splashing. This forces liquid oil up through the breather tubes and into the air intake system.
On turbocharged vehicles, worn internal compressor seals can leak pressurized engine oil directly into the intake charge pipes, causing heavy oil pooling inside the pipes that eventually seeps out of the joints.
Severe internal engine wear allows combustion gases to bypass the pistons and pressurize the crankcase. This overwhelms the PCV system, pushing large amounts of oil vapor into the air intake.