Well, this whole what is a "true" synthetic is a canard. It was something several years ago, but means virtually nothing today. Group III "faux" synthetics offer many great qualities, including a couple that are BETTER than Group IV PAO "true' synthetics. Actually, the best synthetics today are a mix of Group III and Group IV synthetics, taking advantage of the special qualities of each group. It is virtually a total waste of time playing the Group IV vs Group III thing. Even many of Amsoil's product line has a combination of Group III and Group IV. Even Amsoil is smart enough to not play in the Group III vs Group IV game, knowing full well that there are special qualities to each group that brings something to the party. And even Group IV "true" synthetics are made from natural gas. Well, actually ethylene gas that is derived from natural gas. Group IV PAO is just another variation on carbon chained molecules. Every base oil from a Group I thru a Group V is a carbon chain structure.
And, Amsoil makes NOTHING. They do not make one drop of synthetic base oil. They are a blender. The purchase base oil from Mobil, Shell, and other sources. They get their additives from the standard suppliers like Infineum and Lubrizol. Being a blender is not a bad thing, but it is what Amsoil is. Schaeffer, the brand I use the most, is also a blender. They have been putting out very good quality oils and greases longer than anyone else in N. America, since 1839. They even put together a special open gear lube that Shell could not replicate to meet a customer demand, so Shell just has Schaeffer make it for them.
And since the additive package makes up to 25% of most every motor oil, it is as critical as the base oil itself. I would take a Group III "faux" synthetic with a killer additive package long before I would consider a Group IV "true" synthetic with a mediocre add pack. And TBN is just one characteristic of the add pack. There is a lot more to the mix. Amsoil like to pile on the TBN to promote the longer oil change intervals. And TBN's main purpose is to control acid buildup in the oil. When you look at the other aspects of the add pack, it is not something that is heads and shoulders above anyone else. Zinc, Moly, Calcium, Sodium, and other aspects of the add pack and how the levels compliment the characteristics of the other components, including the quality of the viscosity improvers, has as much to do with motor oil performance as does the base oil.
Actually, some of the best oils are one that are a Group III/IV combination with a high quality add pack.
And if you do your research, oils and lubes made from whale oil are still classified as "true" synthetics, but you can bet your bottom dollar that I am not putting whale oil in my pickup. But regarding the Group III "faux" synthetic vs Group IV "true" synthetic.......
Performance of Base Oils and Future Trends - The Evolution of Base Oil Technology - Part 3