If you are leasing a vehicle, then the BETTER mileage parameter is definitely more important as well as cost effective. You just do not care how long will engine last on a car that you will only operate for 24,000 to 36,000 miles. But how many gallons of fuel you will burn will make a difference.
If you own your vehicle for the long haul, or indefinitely, then SAE 5W-50 is absolute must!
That is why SynLube Lube-4-Life is available ONLY in that grade, since we guarantee 300,000-mile service life from engines that were engineered to last no more than 100,000 to 150,000 miles. It makes that much difference!
But due to unique colloidal technology used in SynLube™ Lube-4-Life™ you still get mileage benefit that is 1.6% to 2% better than even SAE 5W-20 petroleum motor oils that are now used by OEM's.
Actually, SynLube does make a 5W20 oil on request but only guarantees it for 70K miles.