Hey nibbles, congrats on your 11' JEALOUSSSS
As for the grind, it happens to new MT drivers like myself.
NEVER try to go into reverse while car is moving even SLIGHTLY. In your case I think you tried to shift the knob when it wasn't in correct placement, while OFF the clutch. It's happened to me once in a parking lot.
As for oil, dealerships use Mobil 1 Synthetic but I read on many Subie forums that there's MUCH better synth oil for the same price as Mobil 1. Penzzoil Ultra is good, Castrol synthetic also and Mobil 1 are all I've tried(5 quart jugs). Subaru dealerships have the OEM subaru oil filter($6.25), it's solid blue and comes with a silver seal ring. Looks like a circle key holder.
Oh and NEVER go to the dealerships to service. They charge 65+ All the oils I mentioned above + filter should run you $35. Then have your buddy help you change your oil.
If you don't have a lift or tools, go to a cheap service shop and have them do it for $10.
$65 + < $35 DIY or $45 Screw dealerships