Engine oil IDEMITSU or Motorcraft

You don't need to add any more additives to modern day oils. You said you will be using Mazda 0W-20 oil? This is one of the best 0W-20 oils on the market. It is made by Idemitsu for Mazda. It has very high Viscosity Index of 221. Most 0W-20 oils are around 170. This means it is one of the lightest 0W-20 at start up. This is what you want in the winter.

found some FORD Super Duty Diesel oil 15w40.....seems to work pretty well! ford aka motorcraft

ive always used motorcraft diesel oil. 10w30 in the winter and 15w40 in the summer.