I noticed a slight reduction in piston slap when I put Rotella T6 5w-40 in our 1999. Though, it has a pretty bad case of it.
I run a thicker oil for a variety of reasons. First, Rotella T6 is a very stout synthetic widely available for a very reasonable price. Second, the bearings on a higher mileage vehicle will have opened up a few thousandths over the years and the thicker oil provides better protection. Finally, I already use it in our 2011 Impreza and my 2009 STI, so I buy it in bulk and it's already around.
Both of our other Subies run great with it (with UOAs to back it up). I first only used it in the Forester for one oil change because it was leaking enough to make it cost prohibitive to keep adding that oil, so it was filled with cheap-o Autozone 5w-30 until I fixed all the oil leaks. I'm still on the first oil change after the repairs.
Everyone has their own opinion on what oil is best for a particular engine. I go off experience and hard numbers (UOAs) from myself and others to make my decision, and you should too.