Just an FYI - A repeat from another post..
The LBZ motor should use SAE CI-4 grade oil. 15W40 should be used for ambient (outdoor) air temperatures greater than 5 degrees Farrenheit. 5W40 should be used if ambients are below 5 degrees F.
The new LMM motor (January 07) should use CJ-4 grade. This grade is backward compatible for the older engines (LBZ, LLY, LB7). It has a lower sulfur content which protects the LMM aftertreatment.