By-pass filtration of engine lubricating oils is now a widely used technique for controlling the cleanliness of the oil during a vehicles service interval. The two common by-pass filtration techniques used by the automotive industry being barrier media filtration and self-driven centrifugal oil cleaners. In general it can be said that the fundamental operating principles of a centrifugal oil cleaner are poorly understood by others. The aim of this paper is to raise the general level of understanding of centrifugal oil cleaners and to aid lubrication system designers in the comparison of these devices with other filtration techniques.This paper initially discusses the reasoning behind by-pass oil filtration and then goes on to assess some of the fundamental operating parameters which need to be addressed when fitting a self-driven by-pass centrifugal oil cleaner. Methods for determining the cleaning performance of traditional barrier media filters are adequately covered in the SAE standards J1858, HS-806, J1985 and ISO standard 4572. A standard method for evaluating the performance of a centrifugal oil cleaner is not well defined however. This paper sets out simple procedures and analysis techniques for the representative comparison of by-pass barrier media filters with centrifugal oil cleaners, thus enabling meaningful and repeatable testing to be performed.