Yes, it is OK to mix mineral and synthetic oils. One of the early synthetics used was a Polyalkylene Glycol. This was totally incompatible and would gel when mixed. This has not been used for years for automotive lubrication. All common synthetics used for engine lubrication now days are a Polyalphaolefin (Mobil 1) or a Dibasic Organic Ester type (AMSOIL). These are fully compatible with conventional oils. In fact Golden Spectro and AGIP Sint 2000 are mixtures of mineral and synthetic oils. It is always best to mix oils with the same rating (SG).
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