You wrote...................
"Now, this is also what I have observed on my car (300CE M103):

1. 3 of the spark plugs are wet with motor oil. Changed new ones and happened again.

2. 1 Quart of Oil every 500 miles

I've done a head job about 6000 miles ago and valve stems were replaced but not the guides....."

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Hate to say this, but it looks like you answered your own question. Doing valve seals is good but not always best.
I have had some cars here with seals replaced by other shops and car still had high oil consumption, found in all cases the guides where worn and in most, the new seals distorted and or not still gripping the guide and floating on the valve stem.
During throttle closed idle, the vacumm/vacuum/vacumn (what ever!) is high and will suck oil down the intake guide, as well as the very high vacumm (Here we go again.) that occured as you where slowing down with throttle closed before coming to idle.
( Called "tickover" by us old UK 'fitters'..) exaserbating the problem of plug oil fouling.
Run car warm, up to 100 kph and drop it into next gear down and let it coast down to 20 kph or so and then give it WOT and watch for tail pipe blue smoke in the mirror....This test is best demonstrated by travelling in another vehicle behind your own so you can observe the amount of smoke emmited. Think you might be quite surprised .
There is a strange phenomena that occures with valves in loose guides that I will post to TECH in a different heading.
However, it seems you are a candidate for valve guide replacement.

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