that is cool with me. but i would be looking at the part's more then the lube, if you used a hi quality lube with the proper cert's on it you should be fine.
synth lube is getting kinda like the aditive market, alot of false claim's have been made by alot of non-full synth lube maker's. i hate this, it make's a bad name for the good maker's and there is not much that they can do about it. the court's have ruled that you can use the synth name but not have 100% full synth in the lube.
there are 2 type's of synth lube, one is a man made fully synthectic base oil, like amsoil and mobil1, the other's are a highley refinded dyno juce stuff. it has most but not all the property's of the man made synth and is cheeper to make.
in other word's full synth in my book comes from a test tube! not a refinery. but the court's have said no, the stuff from the refinery is the same stuff, or close enough to be called it. kinda like generik drug's they are chemicaly the same but may not preform exactly the same.
as for your tranny, a bad case just hair warped, or a bad gear that was not tempered right can give you what you got. a slightly warped shaft too. tranny's have a very tight tolerances and .100 of an inch can cause a problem, i have seen this too many time's. this is not your falt sometime's you just can't tell with out some very expensive eqepment and alot of money to pay for the testing.
we had a T-10 roadranger tranny puck out and the lube was right, the clutch was right, every thing was right. but after a closer look the manufacturer determend that the case was not line bored right. it was off by only .150". that was all it needed to crap out. there is alot of stress in these box's, so it dose not take much to kill one.
but hey you can belive how ever you want and so will i. and that is cool, cause that is one of the great thing's about the good old USofA! and you are not dumb or anything like that. you just have a diferant opinion and i do respect that.