I was just googling high mileage oils when I can across this page,
https://www.machinerylubrication.com/Read/31414/high-mileage-oil the following is about 3/4 down, just thought it was interesting what they say about the blend, I took it as kind of the best of both worlds.
Conventional Motor Oil
Conventional motor oil begins as crude oil from the ground. It is either solvent refined or hydrotreated and can be classified as Group I or Group II, under the API base oil classifications.
This type of mineral base oil carries additives well due to its molecular structure. Yet, it does have a shorter lifespan when compared with synthetic oil and will have a lower viscosity index than a true synthetic or Group III mineral oil.
Synthetic Motor Oil
Any motor oil with a base stock consisting of a Group III, IV or V oil would be considered a synthetic oil. Although Group III oils originate from a mineral oil, they undergo such high refining that they hold properties very similar to a true synthetic oil.
Synthetic oils generally are more resilient against temperature extremes and sheer than conventional motor oils due to their molecular structure and uniformity. One of the drawbacks of synthetic oils is that although they tend to have higher oxidative stability, they may not hold additives in suspension as well as a less refined oil.
Synthetic Blend
A synthetic blend is a combination of oils, such as a Group I or II oil mixed with a Group III, IV or V oil. Conventional and synthetic oils may be mixed so the mineral oil can hold additives in suspension while relying on the synthetic for scrubbing capability and oxidative stability.
A review of safety data sheets reveals that a Group II and Group III blend is the most prevalent, with few manufacturers using a Group IV or V oil as the synthetic in the blend.