Conventional vs. synthetic will likely play a role, too. I bet you'd see a difference switching to a good synthetic, even if it's 10w-40.
However, it's always a good policy to pick the lightest oil that'll do the job.
Mobil 1 5w-30 is a good pick.
Feel like shelling out a bit more for a bit of an upgrade? Renewable Lubricants (RLI) BioSyn has been said to blow everything else out of the water, and they make a great 5w-30.
Other alternatives are Amsoil ASL 5w-30, Red Line 5w-30, Pennzoil Platinum 5w-30, and Castrol Syntec 0w-30. All of these have very long and very good track records.