Originally Posted by DGXR
Most motor oils claim that they are compatible with all other conventional and synthetic motor oils. For example, here is a statement from the Mobil website:
"Mobil 1 full synthetic motor oils are compatible with conventional oils, oils for high mileage vehicles, synthetic blends and other full synthetic oils. Engine flushing is not required."
Yes if the oils are API licensed then they must pass ASTM D6922 and will be miscible. The mixture must not cause harm to pass that test. However, that is not the whole story. Not causing harm is all it has to do, there is no guarantee that the resulting mixture will meet any standard, license nor approval that any of the individual oils may carry.