Im running 15W-50 mobil one full synthetic. Engine builder recommended it, from what i remember he opened up the oil passages some and im sure he knew what the tolerances were. Having the engine builder tell you what to run is a good basis to go off of. But if you cant talk to the engine builder it makes it a little harder to make a decision, honestly 20w-50 or 15w-50 should be no problem even in about 30 degree weather. I used to run 20w-50 mobile one and had to change to 15w-50 mobil one because they discontinued the 20w-50. I have the 15w-50 in year around and even in the 30’s it does perfectly fine. All depends on engine tolerances
