Apparatus and method for adding one or more additives to an engine lubricant, whilst the lubricant is circulating within an engine having an engine lubricant filter with a by-pass valve which opens when the pressure of the lubricant passing through the filter exceeds a pre-determined back-pressure value in which the apparatus further comprises an additive chamber containing one or more additives operably connected to the by-pass valve such that when the by-pass valve opens at the pre-determined back-pressure, at least some of the lubricant is caused to flow through the additive chamber and contact the one or more additives.