metallic debris in oil filter

I wouldn't worry at this point. Do another oil change in 25 hours. There is also a far better way to inspect oil filter elements. On the filter medium the particles are hard to see or diagnose at times. 

Take your cut out filter medium then drop it in something like a pickle jar. Pour in about 1 " of gasoline. Put the top on and GENTLY shake it like a washing machine. Now take a paper towel and place it over a small container. Remove the filter medium from the jar and you can examine it, but it will be very clean. Now pour the residue left over in the jar over the paper towel that acts like a filter. Anything and everything will jump out like a red flag and it's easy to see. If you see something it's easier to determine what it is. You can use a magnet here if you want to.

thanks a lot for your answer. In your expertise this non magnetic debris  can originate fron lifter surfaces or these are solid and magnetic (i.e. ad some steels)?

From losing a lifter once myself this isn't from a lifter or it would mean the hardness coating for that lifter has been compromised and would just disintegrate and trash the engine in fine metal dust everywhere.

Hi Nicola

The lifters are magnetic particles.  Most likly it may be a bit of material from the cylinder coating on the lower part of the cylinder barrel.  It may also be some material from the piston itself however that is a very small particles.  Any metal parts from the gearbox would be magnetic also, they are all steel.  

The only way to know for sure, oil testing.  It is common and they wiil determine exactly what metal it is  Once that is done then you have an idea where to look. 

Cheers