I have a 01 legacy GT limited; with 120,000 Km. 3 weeks ago put Mobil1 5W-30. Piston slap still there. Lasts about 2-5 minutes until engine comes up to normal operating temp.
I have used synthetic's in all cars I own. My engines last well over 300,000km. I use Mobil 1 because; its seems improve the longevity of the engine, and somewhat improves gas milage.