How to Fix or Identify Engine Oil Leaking Into Spark Plug Well?

Some time ago I realized that one of the spark plug cap out of the four had some oil leakage stains so i decided to check it out.

I'm no expert in this but if you ever come across such a condition, the most basic things to check is are the valve rings/washers and the spark plug holder's washer or also known as 'O' rings. The spark plug holder on a Toyota Unser looks like this in the photo below.

In my case, the worn out O ring on the first spark plug holder was the cause of the leakage of engine oil into the spark plug chamber. A good condition O ring should be square edged( as shown in photo) and not round which means it's worn out. In the end a simple change of an O ring stopped the leakage.

Some random notes:
1) NEVER add engine oil when a spark plug is removed. Always add engine oil after securing all the spark plugs.
2) Spark plugs shouldn't be covered in engine oil. If it is covered in engine oil, it could be an engine oil leakage.
3) Engine oil present in spark plug well could also cause noticeable symptoms to the car such as underpower of the engine, misfires, white smoke fuming from the exhaust with a strong burning smell etc.