Since you're using Mobil 1 15w50 the easiest would be to mix in some of their 5w50 to help lower the winter rating. I've found in our diesel farm tractors that switching over to a full synthetic or even a blend really helps them crank over a lot easier during the winter. Do you have a block heater that you can plug in?
You can also try adding a quart of MMO as they recommend it during the winter months for easier starting. I've used it in my Honda Civic and GMC w/ 5.3, supposedly works great for freeing stuck lifters and rings but I haven't experienced those issues but figured a bottle every so often can't really hurt either other than my wallet. It's not something I use all the time either, typically every 3rd or 4th oil change I'll use it.
You can also try adding a quart of MMO as they recommend it during the winter months for easier starting. I've used it in my Honda Civic and GMC w/ 5.3, supposedly works great for freeing stuck lifters and rings but I haven't experienced those issues but figured a bottle every so often can't really hurt either other than my wallet. It's not something I use all the time either, typically every 3rd or 4th oil change I'll use it.