1.1 This practice covers the compatibility of mixtures of hydraulic fluids as defined by Specifications D6158, DIN 51524, ISO 11158, and ISO 15380. 1.2 This practice can be used to evaluate new (unused) lubricant compatibility or the effects of combining new (replacement) lubricant with in-service (original) lubricant in the system. 1.3 To evaluate primary compatibility using this method, the replacement fluid must pass the ISO 13357-1 Stage II filterability test. The original fluid is not required to pass ISO 13357-1 filterability test, Stage I or II. 1.4 Primary testing is conducted on fluid mixtures in 2:98, 10:90, and 50:50 ratios using the ISO 13357-1 Filterability Test, Stage II. 1.5 Secondary testing is suggested when circumstances indicate the need for additional testing. 1.6 This practice does not evaluate the wear prevention characteristics, load carrying capacity, or the mechanical shear stability of lubricant mixtures while in service. If anti-wear (AW), extreme pressure (EP), or shear stability are to be evaluated, further testing of these parameters may be required. 1.7 This practice does not purport to cover all test methods that could be employed. 1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. 1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TB