[Werner]

Kimlinhn ha described it exactly and correctly. Oil always gets dark because it gets residues from combustion in the cylinder. Every combustion that does not run 1: >1 stoichiometry produces a lot of soot.

You see that well in the back of the black exhaust or if you have started the engine with exhaust end in front of a wall.

Soot is oil-soluble, but is only partially filtered out by special filters in the sidestream. This is only available in modern injection engines, as well as the exhaust remains clean.

But there are oils that make more or less residue from your own combustion! Transmission oils, ATF, Rhicinous, etc, leave a lot of residue, two-stroke oils very few. It depends on the additivation.

Synthetic oils leave very little combustion residue, but they are very hard. Simple mineral oil leaves more combustion residues that are soft for it.

Much more important than the type of oil is the regular change to remove the many soot residues out from the motor. These are oils above API "B". A filter retains coarse residue that is sucked in or chipped off hard oil crust. The oil filter prevents wear, but can not really keep an oil "clean". Personally, I would change the oil every 1000 miles / 1 time a year with the filter. Without filters every 500 miles. The few rinsed short distances, the more gerunger is the soot entry.