I remember reading somewhere that the most abuse your engine takes is upon initial startup when it's cold. Granted, there may be other situations where much more abuse is issued - like redlining the engine for a period of time, running it with an extremely low amount or without any oil, running it with extremely dirty oil that has never been changed, etc... but seriously, when you start a cold engine (one that's been sitting for a period of time - a few days, overnight, a few hours, etc.), you want the oil to be able to lubricate all the moving parts immediately as well as continue to lubricate everything adequately once it gets up to operating temperature and not break down or become too thin. If you use an oil that's too thick, it won't be able to adequately lubricate all the parts until it warms up, so during that whole period of time before that happens, parts will be grinding and sustaining unnecessary wear. That's why Subaru recommends 5w30 since they found it to be adequate for most climates across the country. Some people (I believe in warmer climates) use 10w30 and that's probably fine.
Also, here's some information about synthetics for those who may be interested:
There are several types of sythetic oils on the market and the base stock is what differentates them.
The best base stock is polyol-ester (Redline & Motul)
Next best is di-ester (Royal Purple, Amsoil, Neo)
Next is polyolefin polyamine (Mobil 1)
Next is Hydrocracked (Syntec, Valvoline)
And finally regular dyno oils