Engine oil OEM Audi or OEM Subaru

Over here in Germany changing your oil every 3k miles in a normal A4 absolutely tanks your resale value. The only reason to do this frequent oil changes is to hide problems. Personally my car did 30 000km(~20k miles) until 91 000km and then every 15 000km(~10k miles). It's a diesel and almost all longish distance highway trips.

I do 5k/3 months. 3 months usually comes first.

In Canada Audi recommends oil changes ever 15,000km. I always do 10-12,000km because I do city driving. My buddy did every 25,000km on his Mercedes Benz and the thing lasted to 400,000km.

5-6k. It depends on how much you push the car. Hard acceleration and high revs will require more frequent oil changes.

Well, some manufacturers do have way shorter oil change intervals than Euro manufacturers. The Subaru severe driving interval is 3000miles/6 months, and I do lots of short trips in a desert climate so I usually try to be around 3-4000 to stay on the safe side for warranty and whatnot.

I run the 5w-40 in my 2004 Audi A8 and both of my power strokes just because it's handy and the diesel grade oil is overall better than the other synthetic I was running in a 0w-40 weight.

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.