Went over my head too. I know from experiance of actually testing oils to distruction that fully synthetic thins out too much at high rpm and operating temperatures.
Semi synthetic doesn't do it as bad.
How do i know this? Cos at 140mph my tappets are screaming, the bottom end sounds tinny, and when i stop and check the oil its like water and has absolutely no lubricating properties to it.
All this technical explanation doesn't change my view on oils at all, i go by what i've seen.
p.s
viscosity is generally reported in centistokes (cst) and usually measured at 40degC and 100degC.
Dunno about anyone else, but the only time my engine see's 40 + 100 degrees is about 2 seconds after starting up. Then it runs at 150-200 at normal temp. These oil gurus who make this shit should actually get out of the lab and drive the fucking cars!