I guess there is an assumption by most people that Group 3 syns are not real synthetic , in a way that's true. Btw a semi syn is a grp II oil mixed with at least 30% group III.
The argument about group 3 being not real FS began in 1999 when castrol marketed their Syntec Group 3 (hydrocracked mineral) .
Excerpt :
"In the late 1990s, Castrol started selling an oil made from Group III base oil and called it SynTec Full Synthetic. Mobil sued Castrol, asserting that this oil was not synthetic, but simply a highly refined petroleum oil, and therefore it was false advertising to call it synthetic. In 1999, Mobil lost their lawsuit. It was decided that the word "synthetic" was a marketing term and referred to properties, not to production methods or ingredients. Castrol continues to make SynTec out of Group III base oils, that is highly purified oil with most all of the cockroach bits removed.
Shortly after Mobil lost their lawsuit, most oil companies started reformulating their synthetic oils to use Group III base stocks instead of PAOs or diester stocks as their primary component. Most of the "synthetic oil" you can buy today is actually mostly made of this highly-distilled and purified dino-juice called Group III oil. Group III base oils cost about half as much as the synthetics. By using a blend of mostly Group III oils and a smaller amount of "true" synthetics, the oil companies can produce a product that has nearly the same properties as the "true" synthetics, and nearly the same cost as the Group III oil. In fact, Mobil-1 is now primarily made from Group III unconventional base oils, exactly the stuff Mobil was claiming was not really synthetic. The much more expensive traditional synthetics are now available in their pure forms only in more expensive and harder to obtain oils."
So to summarize, there is no need to be pedantic about off the shelf synthetics like Shell Helix Ultra, Castrol Magatec , or Mobil 1 not being "true" FS, their performance is already VERY close to Group IV(Amsoil/REDLINE/ RP) In fact, it has been shown that additives blend in better with the lower base oil. You will be hard pressed to tell a difference at 10000km OCI.
To cap this, let me show you a video.