Good question. So, since Mobil makes both, I went to their web site and looked at the specs. The parafin based 15w-40, Cx-4 rated oils have some pretty impressive specs. For what it's worth, this Mobil oil has a later rating than the syn. Maybe they haven't retested the syn lately. Anyway, I started using syn's several years ago and have noticed improvements over prafin oils. My observations: Parafins stay cleaner longer. This is due to their tolerance for high temp; don't break down as readily as parafin oils. On the other end, for a given weight, syn's have a significantly lower pour point; like today we were talking about -38F pour point for Mobil-1 15w-40. Their molecular attachment (to metal) seems to be better. Initial start ups are cleaner. Case in point. Had a '88 F150, 302v8. After an oil change, lifters would always rattle before the pressure was up, even with my filling the oil filter prior to installation. With syn it never did it again. Another. Had a rice burner 3 cyl diesel LT. Cold weather starts were always accompanied by significant piston skirt slapping (very noisy). Used syn and it quit. What I have done, on all engines (except air cooled) is go 100% for a couple of changes then blend (my own blend, not storebought blend)about 20% syn with the parafin. Seems to work ok. Air cooled get 10% always. 1 change per year; stays clear all year. Supposed to reduce friction which makes engines run cooler with better efficiency. Don't doubt the claims (make sense) but have no scientific nor empirical data to prove it personally. Mark
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