What contaminants does the oil filter not remove?

The oil filter is designed to trap various contaminants that can circulate in an engine's oil, ensuring that the oil remains clean and effective at lubricating engine components. While the filter is efficient at removing particles such as metal shavings, dirt, and oxidation products, it has limitations, particularly in handling certain types of contaminants.

In this context, carbon and soot are products of combustion rather than physical debris that the filter can easily trap. During the combustion process, carbon can form as a byproduct and soot can accumulate, becoming part of the oil composition itself. Since these substances are often too small or too chemically integrated into the oil, they are less likely to be filtered out by a standard oil filter.

On the other hand, water and dirt, metal shavings and dust, as well as oxidation products and sludge are typically much larger and/or more amenable to being trapped by the various filtration mechanisms that oil filters employ. Thus, the filtration capabilities of standard oil filters may not effectively deal with the finer particles like carbon and soot, making this the reason why it is recognized that they are not removed by the oil filter.