So is Castrol Edge.
They actually have quite different formulations. HTHS viscosity values were estimated by setting the base-oil +DDI VI to ~ 130. Noack values were estimated from the CCS values, assuming similar base oils according to the MSDSs. MSDSs also show that Edge uses a larger percentage of the heaver base stock, in agreement with the calculations.
Castrol Edge 5W-30 SP:
- HTHS viscosity: 3.05 cP
- VII content: 4.7%
- Base-oil viscosity at 150 °C: 2.26 cP
- CCS: 4,660 cP
- Noack (estimated): 10.5%
- Ash content: 0.78%
Castrol GTX Full Synthetic 5W-30 SP:
- HTHS viscosity: 3.25 cP
- VII content: 7.4%
- Base-oil viscosity at 150 °C: 2.15 cP
- CCS: 3,900 cP
- Noack (estimated): 12.5%
- Ash content: 0.92%
Estimated VII content and base-oil viscosity of selected oils
Neither is necessarily of superior quality to the other. They are both of non-GTL Group III. GTX Full Synthetic is made from a thinner base oil with more VII added to thicken it, and it also has more ash, possibly meaning a higher TBN.