want to switch to 5w40 where to find it??

[snowmobiler7c]

I have a 03 f250 7.3L that i have had some problems with cold starting this winter. Last year wasnt as bad but temps wernt as low this often. Starts fine when plugged in or above 20degrees. Batteries are diehard 850 or 875cca cant remember for sure. I changed my glowplugs about a month ago since several were out of spec. The glowplug relay is working fine. But when the temps are in the single digits its cranking very slow until it fires a couple times increasing the speed of the motor trying to start. I was told by NChornet to switch from the rotella 15w40 to rotealla 5w40 but cant seem to find it anywhere. I have checked tractor supply, o'riellys, autozone, advance auto parts, and wal-mart. Any ideas?? If i can get it in Conoco is that a decent oil to run? I havent checked yet but my buddy owns a gas station delivery service and uses/sells conoco. thanks for any input
mark

[fordpwrstroke7]

In Texas, Wal Mart has it. Not sure about your neck of the woods.