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?
Oil does not normally get thicker during use. High temp shearing breaks up the molecular chains and that factor combines with some fuel contamination to thin the oil, BUT once the detergent additives are used up, the oil starts to thicken up because small lumps of mostly Carbon blowby contaminants start to build up. Many modern full synthetics are fairly resistant to high temp shearing, so if the injection system is in good order and the engine does not spend too much time at idle or short tripping, the reduction in viscosity during a normal OCI is not very great.
Another factor is the location of the oil pressure sensor unit. In most cases it is just after the oil filter, so if the oil filter starts to block it will result in a slight drop in oil pressure.
Finally, some cheap oils suffer some degree of evapouration when hot, which does result in a very small degree of thickening. That issue tends to only apply to El-cheapo dinosaur base stock oils.
Thin oil = lower oil pressure, BUT thin oil flows faster, so for engines where top end cooling is critical (Race engines in particular), using too thick an oil can be bad news.