I did not intend to offend you, and I'm sorry if I came off sounding like an ass, but I cannot let statements like yours slide. Stating that using Syncrhomesh (which is basically 5W30 motor oil with some additives) in place of 75W90 gear oi, is the only option anyone should consider is indeed absolute and uninformed. Your statement left no room for other options, and made no regard to any vehicle's intended use, modifications, or current symptoms. It could give someone a really bad idea and they could wind up trashing a transmission.
Nice edit. I am glad I copy and pasted before this....
Okay, why did you state that GM Synchromesh was the only transmission oil that anyone should use, ever?
Because of research and personal use. I had a 2006 MS6 which has the same motor and tranny you have in your MS3.. Now, the 2006 MS6 had issues with shifting between the 2006 and 2007 models. This is why Mazda had a TSB for it, and it was done on my MS6 when I had it...
However, I still had a notchy feeling in my MS6 while shifting. I did some research, just like you mentioned, and noticed a lot of it was negated by swapping to GM Synchromesh... My original thought was "Wtf are these people thinking?!", but I did it on blind faith and swapped. Ever since I swapped to it, I had no issues in shifting and I can have 5, yes FIVE, people tell me that my MS6 shifts so much easier than other cars.
Let's fast forward to my current car... A 10AE with 139k on it... It has GM Synchromesh in it too. The car shifts like a friggin champ. I have put it through some scenarios that you would call "extreme" and it holds up just fine. Also, and I hate to name drop, but Shadow102 is a certified mechanic and has been doing it for years now... I will take his word based upon experience and knowledge instead of some internet guy who takes is car out only when it is above 50 degrees....
When it comes to oil for anything there are many things thing to be considered. Most companies that claim "true synthetic" are really not. Most are a class 3 dino oil that has been "hydrocracked" and modified to be a synthetic. This is not only for engine motor oil, but others as well. Some of those additives are really beneficial to trannys, engines, and such. So in reality, you and I are right.
There are numerous things that marketing does for oils, and your AMSOIL guy is guilty of, that make us believe they are better than others. There is so much involved in oils for our cars that we can't even imagine unless you have a degree in it and work in the industry. Haha....
In the end, go with what works for you but don't just brush off someone who has had experience and such.
(edited for grammar)