Some lubrication systems of the internal-combustion engines are equipped with two types of filters, which work serial-parallel. Filter that is placed in series (full-flow filter) passes almost all pumped oil. Second filter, which is placed parallel (by-pass filter) cleans only about 10% of oil. This oil after being cleaned goes back to crank case.

The same way there are oil filters equipped with two filtering elements inside. Such construction provides bi-level filtration, medium and fine cleaning accordingly. The oil flow is divided inside filter with full-flow and by-pass elements. This division assists in selective trapping of contaminations. Thus the oil filtration is improving and filters service life is loner.