Here is a brief lesson on motor oil.
1. When the formulation of an oil changes (such as from SN PLUS to SP), the Noack will change. This is because the Noack primarily depends on two things: (1) base-oil viscosity and (2) base-oil quality. When the formulation changes, the blender will usually change the base-oil viscosity and readjust the KV and HTHS by adjusting the VII content. The base-oil quality usually remains about the same, as blenders are unlikely to resource their base stocks. Castrol typically uses low-end (cheaper) Group III base stocks in their oils marketed for the US (their ILSAC oils (as opposed to their ACEA oils)).
2. We have been seeing that the base-oil viscosity has been getting lower and the VII content getting higher with API SP oils because of the fuel-economy emphasis in SP.
3. Without doing any calculation, you can see that the SP is a lot thinner than the SN PLUS, as the densities are 0.8448 (SP) vs. 0.86 (SN). Likewise, the VIs are 175 (SP) vs. 159 (SN), showing the increased VII content and thinner base oil in SP.
4. You can use
my VII/HTFS (base-oil viscosity) calculator to estimate the VII content and base-oil viscosity of Castrol Magnatec SN PLUS and SP. I get about 7% (SN) vs. 10% (SP) for the VII content and 2.0 cP (SN) vs. 1.8 cP (SP) for the base-oil viscosity.
5. This is my estimate of Noack for Castrol Magnatec 5W-30 SP. Assuming that similar base stocks are being used (similar base-oil quality), Noack × CCS should remain about constant. Therefore, we have 8.9% (SN) × 6010 (SN) = Noack (SP) × 4000 (SP).
This gives Noack = 13.4% for Castrol Magnatec 5W-30 SP, which is on the high side for a dexos1 Gen 2 oil. If the base-oil quality has improved, it could be a little better, but I doubt it's much better than that, as I know Castrol's game in the US.
So, if Magnatec has made your engine consume less oil, it's good for you, but it's not because of the Noack. Perhaps you have a leak or valve-stem oil seal problems, and higher KV is helping you. Who knows. It's a mediocre synthetic, and there are better oils out there. I told you to get the top-of-the line Mobil 1 FS 0W-40 (or Castrol 0W-40), but you opted for a much lower-grade oil. You could have picked the Super Tech as well and saved yourself money, without seeing much difference in the performance.
As for motor-oil anecdotes, always take them with a grain of salt.