The "problems" with Mobil1 and Royal Purple posted in other Subaru forums are from turbo owner's running thin Energy/Resource Conserving 5W-30 and 10W-30. Many of these oils will shear out of grade in many applications. People seem to think that these companies only make 5W-30 when they make many different grades. Mobil1 0W-40 does really well in turbo Subaru's. Royal Purple's web site says their API SN 5W-30 meets fuel economy requirements of a 20 grade and recommend their High Performance Street oil for high performance applications. The new GF5 5W-30 oils (even Mobil1) are actually showing good uoa results in turbo Subaru's.
0W-20's are usually made from better base stocks than 5W-30's. Even so, if a light 5W-30 shears to a 20 grade, is your engine going to explode? Sorry, but one $30 uoa is
not going to answer that question.
Is any of this relavant in a BRZ/FR-S that specs 0W-20?
No! 
-Dennis