Going from memory here (I can confirm later when I get home) the manual calls for a 75W90 GL4 oil, and using the "exact best most expensive high quality gear oil" guarantees you nothing.
I'm going to repeat the words of a highly respected local Suzuki guru & motor racer - he had just reassembled a 5 speed manual transmission for one of my Suzukis, and we were discussing what oil to use, my preference being a fairly expensive synthetic (Mobil One) - use regular 75w90, that expensive stuff isn't doing you any good.
Given the damage described, the cause of the failure was probably not related to your choice of lubricant - the problem was either the quality of the work, or the quality of the parts used.
I should have at home, a pdf outlining a range of tests done on various gear oils, I'll look for it and see if I can post it - it makes for interesting reading, and of the oils tested, my first choice is the Mobil One, followed by a Valvoline full synthetic.
A brief disclaimer - I only use synthetic oils in my transmissions (including differentials & transfer case) - that means I don't use "regular" oils in those components, and also those are the only places I do use synthetics.
