The guy at your local auto parts store must not be too bright of a guy.
Any parts place should carry either 75W90 gear oil or 80W90 gear oil. In fact, that's all manual transmission lubricant (MTL) is!
That's all you need. Either one of those is A-OK. Just make sure there is no synthetic anything in it, even partial synthetics are bad for Subaru MT's, I know. Don't look for anything fancy.