1.1 This practice covers the compatibility of mixtures of turbine lubricating oils of the same ISO VG grade and type as defined by Specification D 4304. The Tier 1 method compares the visual appearances of specific mixtures with those of the neat oils after storage at specified conditions.1.2 If the current in-service oil is causing problems or if circumstances indicate the need for additional testing, a Tier 2 method compares selected performance properties of the mixture and its cons