Hi Charalampos;
I suggest you do the following tests on each spark plug. Measure the electrical resistance from the contact button down in the barrel–where the spark plug contact hits; and the electrode at the firing end. If the electrical resistance is more than 5000 ohms, replace the spark plug.
The oil you’re seeing can be caused by:
1) a block rocker box drain tube–if it’s blocked the rocker box will fill with oil–it will be drawn into the cylinder past the exhaust valve and show up as an oil leak.
2) a worn exhaust valve guide. To check for this, remove the valve cover , position the piston to relieve the pressure on the exhaust valve rocker arm, push out the rocker arm shaft far enough to remove the exhaust rocker arm, then see if you and your mechanic can move the end the exhaust valve to the left and then to the right. I use a couple of flat blade screw drivers to push on the spring cap. You shouldn’t see too much movement.
If there’s excess movement, it’s a sign that either the guide has a build up of carburized oil, or that the guide is excessively worn.
I have attached an article I wrote that addresses Lycoming valve guide wear, a Lycoming service bulletin and a picture of the spark plug resistance check.
Let me know what you find.
Steve
