The Ultra should be based on GTL (gas-to-liquid, PurePlus, Group III+) technology and is the newer product.
While the HX8 should be made with Group III HydroCracked base stock, good but the previous technology compared to GTL.
They would be similar, but the GTL should contain less contaminates like sulphur etc., and the GTL should have a naturally higher viscosity index (VI) which means less polymer viscosity index improvers (VII) would be required in the final product. The difference may be minor, but in theory at least, this should add up to the GTL product having a lower Noack volatility and producing less engine contamination.
Of course, if Shell has excess GTL base oil to use up, it may find it’s way into the HX8 on occasion, but you never know.
Sometimes a good indication is looking for the MB 229.5 (or .51) spec, this requires a Noack volatility of 10% or less. Not all Group III oils make this cutoff, which is why a bit PAO (Group IV) or GTL is sometimes used in their formulation. Is the HX8 MB 229.3 while the Ultra is MB 229.5 ? If so, this says it all. Another example was Castrol Edge 5W40 (Group III) MB229.3, and Castrol Edge 0W40 (Group IV) MB229.5 I’ve heard that the original Edge 5W40 was only a fraction over the Noack limit and otherwise would be able to claim MB 229.5, this has all changed with a recent reformulation and now both Edge 5W40 & 0W40 claim MB 229.5, but you get the idea.
This shows the HX8 carries only MB 229.3 and has a traditional Calcium detergent package for it‘s TBN, while the Ultra carries MB 229.5 and has the more modern LSPI friendly Calcium & Magnesium detergent package.