Assuming a 6 month 5000-mile OCI with 50/50 city/highway usage for both scenarios I would do the following:
- My Volvo has a 8qt sump. I would put synthetic Supertech 5w30 without thinking too hard about it. With that amount of OIL vs MILES the oil will be perfectly fine, probably with very satisfactory UOA. Even the conventional Supertech 5w30/10w30 will do fine in that capacity, and usage.
- My friends Nissan Xterra has 3.5-3.8qt oil sump capacity on a V6. With that small sump, and OCI/usage listed above, I would only use Amsoil or Redline, most likely in 5w40/0w40 flavor. And even then the oil could be sheared quite a bit. From oils available on store shelves - Castrol 0W40, Mobil1 0W40, or Mobil1 15w50 if weather allows.