When we buy a new car, then recommended engine oils are usually Synthetic like 0/30, 5/20, 0/40, etc.
But we do not use these oils for all the life of engine. Right? Why we slowly move to the more-viscose oils (mineral) with passage of time?
Similarly, we should consider that this gentleman was using 75/90 oil in his 2004 MODEL COROLLA.
Now, if he will shift to 85/90, it will be perfectly fine and will bring no harm for gear discs.
And mostly the gear oils are good upto 40K~50K and after that we have to change them.
But good companies like LM, PTT, Kixx, guarantees for the longer life of gear oil. Hence we should not consider the Toyota's oil as bench mark all the time.