Who knows if it will help or not. I don't really see engine oil that is good for lubricating helping quiet piston slap since that has to do with the difference in expansion of the blocks and the pistons and their geometry.
For valvetrain noise it depends. If you valvetrain is noisey because your oil is too thin it might help... if it is loud bc the lifters are bad, a pushrod is bent, or whatever else it might not help.
If you look at the product data sheets... you would find that Castrol doesn't list GTX's specs for HTHS which is practically the most important number to compare for lubrication at operating temp. Kinematic viscosity is easy to perform in the lab, but the actual data is only marginally applicable to an operating engine. They also don't list kV at 40C... Oh well:
Mobil 1 5W30
KV @ 40C: 63.1
KV @ 100C: 11.1
Castrol GTX 5W30
KV @ 40C: ?
KV @ 100C: 10.8
Castrol GTX 10W30
KV @ 40C: ?
KV @ 100C: 10.6
Castrol Syntec 0W30
HTHS: ? but believed to be 3.5
KV @ 40C: 72
KV @ 100C: 12.21
At any rate, without knowing for sure since Castrol doesn't want to release any decent data, I would imagine that the GTX dino oil has less operational viscosity than the Mobil 1 5W30 that you are running. Mobil 1 is usually commented on as being very light, but in fact it was reformulated a few years back and the viscosity numbers are a little higher... this does not mean that it doesn't shear, just that it isn't all that thin as some people make it out to be.
As far as the GTX specifically, I don't know what their target for HTHS is or what that number is. However, VII's (viscosity index improvers) usually increase kinematic viscosity somewhat disproportionally compared to HTHS. So if they were using the same chemistry as Mobil 1 I would assume it has less HTHS since the kinematic viscosity is also lower. Alternatively, if you look at the 5W30 versus 10W30 GTX, the 5W30 has a higher 100C kV... likely due to a similar HTHS goal and the need for more VII in the 5W30. All that being said, synthetics with decent base stocks don't need as much VII as conventional anyway so I would assume the Mobil 1 has more operational viscosity and a higher viscosity index.
As far as the Castrol Syntec 0W30 (German Castrol), it is in fact thicker than most of the off the shelf 30's that you get in the US. To the topic at hand, it might help you. If you were coming up on an OCI and already ordered it, there isn't anything to be hurt by trying it out. It has had very good UOA (if you believe in them
) and has been run in many LS1's with good results. I wouldn't personally try out the GTX for what you are describing.