My engine, M111.960 has a slight tick when it's fully warm at idle.
It usually takes 20 mins of driving or so for it to appear.

I assume it is something with the oil. What could this be? And is it a big issue?

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Smoke Silver 1994 E220 Coupe Automatic
0 kilometers 6 September 1994
100,635 kilometers - 6 September 2010

Noises and vibrations are definitely the most difficult problems to help with over a discussion forum.

Does it sound like valve train noise? Does it sound like it's coming from deeper in the engine? Is there any correlation with oil pressure readings? Does it still persist after an oil change?

It's still there after oil change
And, it seems that once the oil is warm enough for the pressure to drop, that's when it starts ticking.

The ticking sound is the same as when I start the car after it was sitting for a few weeks (before the oil pressure goes up). Except the ticking when hot is much less frequent than that.

So, it's in the valvetrain of what I can tell, and it seems to go away with a few revs, but returns at idle

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Smoke Silver 1994 E220 Coupe Automatic
0 kilometers 6 September 1994
100,635 kilometers - 6 September 2010