In my earlier days of driving and tuning, I've had experiences of oily intake manifolds, fouled spark plugs and oil dirty diesel engines.
Oily intake manifolds and oil dirty engines may be caused by oil leaks or simply by a cracked breather hose to the air filter box or broken PCV hose.
If and when you have an oil spitting breather or PCV regardless of your engine condition, it may help to add an Oil Catch Can before you send back those gasses into your intake manifold.
The purpose of the Oil Catch Can is to literally "catch" and store the excess oil that comes with the PCV or Breather hoses. Depending on your engine condition, the amount of oil may vary.
Here is the DIY Oil Catch Can I made out of a 600ml drinking bottle.
Here, the cap holds the 2 brass tubes of different lengths. The longer brass tube is connected to the valve cover breather hose, and the shorter blue tube is connected to the intake side.
Here is how I have installed my Oil Catch Can on my 2C Diesel engine.
Since the day I have installed my Oil Catch Can, I have about between 100ml to 200ml of oil in it, and have ran close to 6000km since July of 2012.
I hope this helps a bit