I apologise for my show of passion over oil.
15-50 would probably run ok in a hot climate, but with reduced gas mileage. Of course 80-90 would choke it, or kill it with the lack of start up protection. But I am pretty sure that the only place Subaru specs a 5/30 for pretty much all temps is in the US and that is because of CAFE. Elsewhere they spec the oils best for the engine.
By the way, the Mobil 1 T&S is rumored to be the same as or similar to the Mobil Delvac 1, a fully synthetic diesel/gas mixed fleet heavy duty engine oil. Pretty much the holy grail for outstanding protection and long OCI's.
vvk, who is on BITOG, here also, I think, has a 2.5L and has run a lot of different oils, with Redline 5/40 producing the best oil analysis. A lot of Suby owners seem to get good results with Redline.