Use of Silicone or engine sealant is required even if a seal is present.
Important: Ensure components being sealed with RTV are assembled within 20 minutes. Components assembled after the RTV has skinned-over, approximately 20 minutes, will not seal properly.
1. Completely fill and slightly overfill the engine block seal groove with a continuous bead of RTV sealant GM P/N 12378521 or equivalent.
2. Ensure the RTV sealant GM P/N 12378521 or equivalent bead is higher then the rail surface (1) by 3 mm (0.118 in).
3. Using a suitable tool spread the sealant in the area near the oil drain back passages (1) away from the oil drain back passages toward the outboard edge of the rail surface.
This is straight from the Service manual. Very important on step 3 After assembly take a screw driver and verify the oil drain holes are free of silicone.