This is not true, there are vast differences in the additive packages used in all synthetic oils. Proof of this can be found in the fact that some can hold up longer than others. I promote Amsoil, but I do so because of my personal experience using used oil analysis in extending service intervals. Amsoil has been offering a guaranteed 25K mile service/change interval since 1972 and has never had a claim that I know of. The simple fact that they have been able to do so is proof of the viability of their claims. Mobile tried it in the 70's but had to stop because their oil wasn't holding up. Mobile now offers their extended performance oil offering up to 15K mile intervals, but they are not the same. Not at all.
This is actually the only REAL way of truly vetting the performance of an oil, especially if considering taking advantage of extended intervals. IT also serves to let you know how your oil filter is doing as well by letting you know what is actually floating around in the engine. I have heard MANY times, XXXX brand of oil and filter have worked "Fine" for me and never "Had problems"; but without utilizing used oil analysis, you can't possibly know if you had problems or not, you may have just been lucky enough to get rid of the truck before the problems got too bad. Kind of like the guy that says he can tell when an oil is bad just by looking at it or running it through his fingers, sorry, it doesn't work like that.
Just FYI for everyone, I also sell Royal Purple and ship it just like Amsoil, but forthe same reasons why I promote R1Concepts brake parts, while I will sell the customer anything they insist on, I actively promote the products I have the most experience with and that I believe in to be the best from personal experience. Not because of any kind of so called testing results provided by any particular company. It has commonly been referred to as "Hype", especially where Amsoil is concerned, but the biggest hype out there is when Mobile, Castrol, Valvoline and others make TV commercials about cab fleets, or running a charger on a dyno until it blows up (yeah, ok, right); that has more hype to it than anything I have ever seen come from Amsoil. Amsoil was the first and continues to be the industry leader in the development and advancement of the synthetic lubrication industry as a whole. All others are still just following the leader. But that is just my on personal humble opinion.