You would need to provide us with more details for us to do any sort of meaningful diagnostics.

Where did you find the oil on the spark plug?
How much oil was there?
Was it on the outside of the plug, or on the tip or "firing end" of the plug?
Apart from the oil on the plug, are there any other issues with the engine - is it running well, is it misfiring?

Oil can get onto the outside of the plug in different ways - it may have been spilt or there may be a leaking O-ring seal.

Oil on the tip of the plug can also get there in different ways - it can enter the combustion chamber past the valve seals, past the rings, or through the intake - if the engine is firing on all six cylinders, it will usually burn away, but if that plug is not being fired for some reason, a defective coil, maybe the plug itself is bad, the oil can build up over time.

Incidentally - oil leaking past the O ring into the plug tube will accumulate over time and can cause a misfire - so it can be difficult to distinguish between cause & effect when looking at a single occurrence - clean everything up, fit new plugs and see if there is a reoccurrence - check that plug after a week or so rather than waiting too long.

 

I would check your valve cover seals. They tend to leak into the spark plug wells.

Another and possible first thing to check, are your valve cover nut washers. They have a rubber backing and if they're bad, they can cause this problem. I just covered this in another post. You won't find the sealing washers at any auto parts store or the dealership. Not easily, anyway. Go to your local hardware store and buy sealing washers in the bulk washers area. Take a bolt with you that's the same size as your valve cover bolts. Find ones that fit snugly around the bolt so no leaks. Use your old cap nuts and torque to spec.

 

Dear these are my answer in red:
You would need to provide us with more details for us to do any sort of meaningful diagnostics.

Where did you find the oil on the spark plug? at the end of spark plug.
How much oil was there? it's arruond the end and it clear appear.
Was it on the outside of the plug, or on the tip or "firing end" of the plug? firing end.
Apart from the oil on the plug, are there any other issues with the engine - is it running well, is it misfiring?
I found some oil in Injector tube. I decided to swap the injector tube and oil pressure valve, and open the spark plug to check it and I found this oil in first one from driver side nearest spark to the oil engine entry.

Oil can get onto the outside of the plug in different ways - it may have been spilt or there may be a leaking O-ring seal. I do not sure.

Oil on the tip of the plug can also get there in different ways - it can enter the combustion chamber past the valve seals, past the rings, or through the intake - if the engine is firing on all six cylinders, it will usually burn away, but if that plug is not being fired for some reason, a defective coil, maybe the plug itself is bad, the oil can build up over time.

Incidentally - oil leaking past the O ring into the plug tube will accumulate over time and can cause a misfire - so it can be difficult to distinguish between cause & effect when looking at a single occurrence - clean everything up, fit new plugs and see if there is a reoccurrence - check that plug after a week or so rather than waiting too long.
by the way, my VITARA has been swapped their engine from G20 4 2.0L to H25 V6 2.5L.
it goes normal at first, but after some time the engine speed came up and down till be off, it is not staple any more, this came after change timing kit, but the Mechanical told me this lack in air not from timing kit.