Subaru OEMS main and rod bearing tolerances are tight for the potential they have. They were designed for the specific power they came with and daily driving normal habits. Metal expands when it heats up; so this happens more excessively with more power and abuse. Low zinc/phosphorus oils will let you down at this point (reaching the internals physical limits, like a stud being stretched to its elasticity limit, but in this case, it's metals expanding)
Main reason for spun bearings/wear, Lack of zinc and phosphorus. Many off shelf oils contain little to NONE! Only companies currently that have a good amount of it are; Rotella, Valvoline (VR1), Amsoil, Motul, and Joe Gibbs containing the highest amount among many other big companies. Basically these two important ingredients protect your engine and create that barrier between all rotating components having metal to metal contact.
Oil weight makes a huge difference along with the time the engine has been running as well. One hour you may have a 25psi idle and 2 hours have 15psi. Things like this have to be compensated for.
Changing your oil much more frequently will save you many heartaches down the road. Oil breaks down much faster based on rpms, power, heat and oil pressure. Some oils properties may break down at a much quicker rate. (This is where zinc and phosphorus comes into play). For those of you who run oils like mobile one which contains basically none of those two additives, this will break down at a much faster rate. Especially it being synthetic. We personally change our oil every 3,000 miles, or early if doing autocross or any type of racing.
Oil weights chosen depend on the tolerances of your block. Looser = more pressure/thicker weight. OEM, slightly thicker but more pressure is good. Trying to cram thick oil in OEM specs with a higher output oil pump will prevent the oil from flowing efficiently. Your mains and rod bearings tolerances are very crucial. You could spin a bearing on start up. Even wrist pin to piston is very important (even if they come together! They may be off and require a hone/mic)
Fresh oil keeps an engine happy. We'd rather spend extra money on oil then having to keep fixing or replacing the engine.