Shes right, even if she might have heard it on the grapevine. Certainly the age and abuse of the box come into play, there is no question of that, however theres also no question that the shockproof crap destroys synchros. My mechanic (who is different to Kellies mechanic incidentally) said the exact same thing when I took it to him for the first trans service after I bought the car. He drained it and put in conventional oil (aka not the blue shit) which made the synchro issues more noticable. Basically the redline stuff cushions the blow when the synchros dont mesh. You feel it more while you have conventional oil because it has a different viscosity and density. When you have the shockproof stuff, its still doing the damage, u just dont feel it.
There was even a notice on the redline website for a time which advised NOT using shockproof on any synchromesh gearbox. Its pretty much meant for dogboxes (not sure about straight cut boxes).
In either case, my box is shagged. I'll be getting a replacement 33gtr S2 box but I sure as hell won't be using anything that looks like smurf diarrhea to lubricate it...
-D