Nobody is saying you can or can't put whatever you want in your transmission. Personally I take advise that is given to me, especially when the person giving the advise is speaking from experience. I grew up bring around tradesmen. The first thing the apprentice learned was to watch, listen and follow the experience the journeymen and masters of the trade were showing them. I'm not saying that a new guy may not have a good idea or point to make, it just needs to be presented respectfully.
In my old transmissions I have the best luck using mineral based 85-90 gear lube. In my work trucks (Sterlings) I use synthetic gear lube which happens to be what the factory put in them when they were new.