I certainly agree that the add pack could be similar, if not the exactly same. From comments previously made by oil formulators on the site, something like 0W20, 5W20, 5W30, 10W30, may be developed as a matrix solution, where one add pack is developed to work on all viscosity grades, so as to save on development costs.
Usually the most difficult viscosity grade is tested to pass, and once this grade passes, the read across rules allow the easier viscosity grades to pass without retesting. So saving on development costs. Also saving on supply chain costs, as only one add pack is need for all viscosity grades.
I'm no expert on read across rules, but if a particular add pack at a certain dose rate, allows a thin oil of a certain base type to pass a wear test (real lab test). Then the same add pack, at the same dose rate, in a thicker version of the same base oil type, has also passed the wear test (without the formal lab test). A read across pass for the wear test.
Obviously there are rules, of when this can and can't be done. With a wear test going thicker will always offer more protection, and so there is no risk in giving an untested pass, if everything else is kept the same. However with winter (W) cold cranking tests, going thicker will reduce CC, and so every grade will need to be individually tested.