I say this everytime one of these threads is posted.....: per SAE oil grading criteria, 10w-30 is tested at -11F for cold cranking and -22F for cold pumping. Therefore, -11F, if you believe the SAE, is the theoretical threshold at which you'd ever want to start an engine with 10w-30 oil in a crankcase. That means, per cold cranking simulator testing, an engine with a positive displacement oil pump should crank over, and the oil will pump. Most manufacturers place the safe limit at 0 degrees F. Will 10w-30 kill an engine during your average continental US winter? There'd be a whole lot of dead engines if that was the case! Is it optimal? Obviously not. Synthetic oil is a whole different ball game. If you look at published specs, many modern day mostly PAO based synthetic 10w-30 oils will pass at a minimum, the SAE MRV/cold pumping test for 5w.