Engine oil Mannol or OEM Subaru

Yes, it is good advice. When a new engine is breaking in, there is a lot more wear metals. It's not uncommon to see metallic flakes or shimmer in the oil during your first oil change. Parts wear/bed in. That's why there is a break-in period on almost, if not all new cars.

I did my first oil change at 1k miles on my BRZ; I saw a lot of metal shavings come out of the oil. The oil also had a metallic shimmer to it. This is totally normal, but I want to get rid of all of that so it is not circulating around the engine post-break in period.

OP I have the same car and engine as you, I've been using the cheap Mannol stuff, whichever is cheapest. Also whichever the cheapest oil filter is from ECP at the time. Car is currently on 120k and I've been doing this for at least 5 years and it hasn't eaten itself yet!

You can actually get decent quality oil which is not from an expensive brand for pretty cheap, particularly if you buy a 20l jug.

For example, Mannol offer fully synthetic oils which meet OEM specifications

Been using Mannol 5w30 Full Synthetic for past 35k miles now without any issues.

For the UK, cheap and good: Mannol Fully Bi-Synthetic Engine Oil Longlife 3 5w30

The car runs well, no problems starting up, no significant oil burning.

I use Mannol 7715 on my 3.0tdi

My '15 Forester will burn a quart of oil every 1000 miles if I run 0w20.

I switched to 5w30 and it's all good for 5000 miles

I use the mannol 5w30, it might be cheap but it's still just as slippy

Having had multiple Subarus that leak oil like crazy (they all do when they get old) the leaks are most likely too much for the stop leak additive to fix anyway.

I have a slow oil leak (something like a quart every 1,500 miles). Dealer quoted just under $3,000 for the fix when it was 17 years old with 110,000 miles. I'll just buy the occasional quart of oil.