Im looking into buying oil in larger containers than quarts. (gallon, or 5 gallon if i can find it)

I want to buy either the Shell Rotella Diesel oil, or the valvoline in 1 or 5 gallon containers. (does anyone know of local yocal places to buy/order it from?) i have a truck parts store, autozone, walmart, farm and fleet and a few other places...

Who changes oil weight from winter to summer?

i have been running a 10-40 for quite some time now, but im wondering if i should run 20w40 for summer?

If anyone knows of some pros/cons for switching weights in different temps that would be great. (also if anyone knows where to locate the rotella or valvoline oils)