Oil on one side of the air filter.

[Dankata]

Okay I've just noticed that there is oil on one side of the air filter, there is also oil all over piston 4. What can cause this? I was thinking something like rocker cover? piston 4 leaking? or spark plug fault?

 

[Cupman29]

Piston 4 as in the opposite end of the block to the air filter?

How can you see the piston, have you stripped the head off?