zz $ p% S0 D. H0 X }# d+ K2 x& a( wTypically engine Manufacturers specify 10W-30 lubricants down to around -15F and 5W-30 lubricants to around -22F. These are continuous temps so spasmodically below a little won't be an issue. As others have alluded to though it may "test" the battery and starter a bit!