The present invention relates to a method for converting a full flow oil and/or fuel filter system into a bypass filter system comprising the use of a retro-fittable bypass filter configured to directly replace the pre-existing full flow filter, without the need for additional hoses or mounts etc. as in existing methods. Also described is a filter device for converting a full flow oil and/or fuel filter system into a bypass filter system comprising: an inlet for allowing fluid into the filter; a plurality of screens for filtering particles from the fluid; a cellulose element for filtering moisture from the fluid; an outlet for allowing fluid out of the filter; and a bypass for fluid to bypass the cellulose element and at least one screen and exit the filter, wherein the bypass filter is configured to directly replace the pre-existing full flow filter.