The 20wt oils have been used by Ford and Honda for over ten years and there have simply been no durability issues - even in taxi service. If you're referring specifically to Amsoil, their 0w-20 oils (they make three different ones), will work just fine here. If you're sticking to a maximum service interval of 7500 miles, the more economical OE or XL Series oils are the way to go. These are basically the same product except that the XL oils use a higher additive "treat" level. So they're good for 10,000 miles/6 months vs a nominal 7500 miles/6 months for the lower priced OE line.
I use the top tier, Signature Series oils, or the Series 3000, 5w-30 (a HD, gas/diesel oil). However I'm also going 15,000 miles between changes, which many people don't feel comfortable with for a vehicle under warranty.
I'd actually be more concerned about the durability of the CVT, since its fairly new technology. The fact that it holds 12.5-13.0 quarts of synthetic ATF does make me feel better, but I haven't seen how they hold up for hundreds of thousands of miles yet. Hopefully they were over tested by Subaru during development!