I somewhat buy this, but if it's really that important you'd think Subaru would make it more readily available and at least educate dealership parts/service departments of it's existence.
I'll give you my local dealership as an example. For a number of years they had and used Extra-S. It's easy for the dealership to get. That's where I got mine. Then they decided to save some money and time so, they started to just use the gear oil from the 50 gallon barrels. What do they care if the syncho's go bad in the tranny's? You can't prove it's their fault and they make good money replacing transmissions.
I thought that Extra-S and Gear 300 felt about the same.