Group III 'severely hydroprocessed' oils have characteristics much more similar to fully synthesized synthetics than standard Group I mineral oils. Group III and Group III Plus base stock oils also must naturally (before additives) have high viscosity indexes. I'm not sure the term "Synthetic" fully applies. In industry they are called hydrocracked or Group III oil and are respected for their additional performance. Note that Group II oil are also hydrocracked.
http://www.zddplus.com/TechBrief10%20-%20Oil%20Base%20Stocks.pdf http://www.machinerylubrication.com/Read/533/base-oil-trends
