The Forester is not a completely different car and yes they have a turbo model. Every note that SoA puts out about synthetic is different than the last. Note that your manual says that synthetic is required for optimum engine performance, not that it's "required" full stop.
And who's definition of synthetic are they using? Not the API's because they don't have one. And replenishment with 5W-40 conventional is allowed? There's NO such thing! In an email newsletter, SoA said that the new Impreza requires 0W-30 conventional. No such thing as that either and the Impreza specs 0W-20 synthetic. The Outback's manual says synthetic may be used but you have to stick with it if you switch to it. 1980's logic.
I'm not suggesting that you run only conventional, but a 3k mile run isn't going to hurt anything.