5W-40 and 15W-40 have the same high temp viscosity rating as in the 40 designation. Synthetic also tends to coke less at higher temps such as in the turbo than dino oil. the lower number designation is the viscosity rating when cold thus the 5W gives easier starting in cold temps. I think the syn. is a good choice if the cost does not bother you. You still should do 5,000 mile oil changes.