Just changed from Mobil Drive Clean Plus HM Blend 10w-40 to Castrol Syntec Blend in 10w-30. I thought that the thicker Mobil would protect the small sump Nissan 3.0 (4 qts total) better. I found out that the 40wt was too thick for this motor (even though Nissan rated it for a 40wt) in the long-run. The Syntec Blend is a bit thicker than the normal 30wt but a good bit thinner than the 40wt. What OCI would you all recommend? I drive about 15 miles round trip. Thanks !!