These hydraulic oils are based on the same high quality mineral oil used in the Viscol Hydraulic Oil range with the exception of the additive pack being zinc free. The additives they contain can affect the filterability of oils. Many conventional oils are formulated with zinc based additives which can become instable in the presence of small traces of water and therefore cause filter blockage. Viscol zinc free HVI Hydraulic Oils incorporate a Viscosity Index Improver to achieve viscosity indexes in excess of 150. This enables them to flow freely at sub-zero temperatures whilst at elevated temperatures their viscosity is sufficient to maintain optimum operating conditions.