A method of relating OEM filtration cost to oil cleanliness and the resulting effect on the reliability on mature mobile equipment hydraulic systems is investigated. A rational for determining the fluid cleanliness to achieve minimum mean time between failure and guidelines for assessing the cost of improved filtration to meet that objective are presented.The purpose of this paper is to review specific portions of recent studies relating to vehicle reliability as affected by hydraulic component/system failure. In these studies fluid cleanliness was measured using the proposed ISO 4406 solid contaminant code and reliability measured as average time between breakdowns. A relationship between specific levels of fluid contamination and system reliability was established. It then followed that if that relationship could be demonstrated, system cleanliness levels could be selected by the OEM with reasonable confidence that extended mean time between failure (MTBF) would result from the choice.