Engine oil RAVENOL or IDEMITSU

I am using the Ravenol for last 3 changes, oil consumption at 5K change intervals is less than 1/2 quart (about 1/4 qt on last change) and hoping this may lead to less carbon build up.

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.