I hear a lot about using a thicker oil to increase the oil pressure. In my 2005 Silverado 5.3 I have noticed the opposite effect. When I used to use Amsoil I always noticed my pressure drop as the oil thickened over the 15k OCI. I recently changed my oil to Castrol 0-30 which we all know is thick for a 30 grade, and sure enough my pressure has dropped, just like the thickened Amsoil. I'd say the "thick" oil drops my hot idle psi from 39 to 37 and my 2000rpm highway cruise psi from 58 down to about 52. Is this normal? Any other oil info for this engine?