10W30 & 20W50 are very stable oil viscosities. 5W30 and 10W40 are less stable, because of the wider viscosity range. (20W50 has a wide range, too, but starts out thicker and its Okay). Oil shears down as it ages, and this is mostly what your oiil filter filters out.
Check out the Chevron Delo 15W40 or Mobil Delvac 1300S 15W40 "universal grades". They are certified for use in big rig trucks and cars and are relatively stable oils with excellent additive packages. If I didn't use synthetic , I'd use one of those oils. Wal Mart sells it for $5.88/gal all day every day.