Keethos said:
But would it be fair to say that using 5w30 would last a bit longer than 0w20?
No, the viscosity grade has nothing to do with how long it lasts. The 'specification' of the oil, e.g. A1, ACEA stuff, that can dictate longevity. For example, BMW do LL-01 and LL-04. The latter is designed for less frequent oil changes, however it's not as good for your engine. You're better off using LL-01 and changing more frequently. Unfortunately things like oil choices very rarely come down to the engineers behind the engine - they're forced to do certain things by the marketing teams, e.g. fuel economy and service frequency, to try and help sell cars. It's often not the best thing for the vehicle itself.
Talking about specifications, I need to get hold of the datasheet for the proper Subaru/Toyota oil and compare it to other 0w-20 offerings on the market (including the Toyota stop/start oil advertised on ebay). Not all 0w-20 oils are the same, which is why when you walk into Halfords you may see the same grade of oil, same manufacturer, but some say "For BMW" and others "For Ford". It's not just a marketing thing in this instance - when you compare the actual datasheets for the oils they can be quite different.
I would recommend choosing an oil as close to the OEM recommended if you wish to retain OEM performance around fuel economy, emissions and wear. Once i've got the datasheet I will create a thread discussing it and comparing it to other oils out there on the market. I'm not 100% sure, but I believe that you don't HAVE to use an OEM oil to retain your warranty, but it has to be of OEM-spec, which should be easily achieved.