Forane® 427A continues to be installed in many systems running with mineral oil, so an oil change may not be required when retrofitting an R-22 system with Forane® 427A. However, HFC refrigerant blends are not miscible with mineral oil, and system layout is critical when determining whether to change the oil to POE or to add some POE to improve oil return. Close coupled systems or systems where the compressor is in a favorable position will generally facilitate adequate oil return. Therefore, technicians must examine and evaluate each aspect of an installation that might inhibit proper oil return. Vertical risers, evaporators positioned below the compressor, or systems with receivers, generally present challenges. . An oil change to POE is also recommended for systems designed to lubricate compressor bearings by pulling oil from the bottom of the oil sump, such as with Danfoss SM scroll and Trane® 3-D® compressors. Either changing the oil completely or adding 15% to 20% POE in these systems is recommended. Check with the compressor and equipment manufacturer for the correct and approved lubricants to use with a particular system.