Finding spark plug oil on threads points to a few suspects and is generally a cause of concern for your car. It can lead to fouled spark plugs which in turn can lead to engine stalling, misfires and poor gas mileage. Have a look at some common causes for having oil on the spark plug threads.
1. Bad Gaskets or Seals
Bad gaskets will allow extra oil to slip in the combustion chamber and drown or short the spark plugs. The most common problems come from the head gaskets and the valve seals. These can be cracked or loose and allow oil to flow where it shouldn't be.
2. Damaged or Worn-out Piston Rings/Cylinder Walls
This is more common on older engines. Wear and tear can cause the pistons or walls to wear out, which in turn will let extra oil into the combustion chamber. Worn pistons can be a problem on newer cars as well. This can be attributed to friction between them and the cylinder. The leading cause for this is inadequate oil levels.
3. Bad PCV Valve
Having a valve that is damaged or plugged up can cause a build-up of pressure in the crankcase that will lead to oil getting past seals and into the combustion chamber. This can be the result of over-throttling the engine in idle or just normal wear and tear.