In the good old days when there were just Group Is, Group IIIs and PAOs, full minerals were 15Wxx and 20Wxx, semi's were Group I/III 10Wxx (except in the US where high Noack all-mineral 10W30's were the norm), full (or very rich) synthetics were 5Wxx and anything that was 0Wxx was mainly PAO based. It worked this way because for each successive reduction in W number, you need better, higher saturates base stock to make the appropriate CCS spec (or in Europe, the Noack spec). Minerals were cheap and nasty, full synthetics were good but expensive and semi's were somewhere in between.
Then along came Group II base oils especially in the US where they totally pushed out Group I. Now 20Wxx, 15Wxx, 10Wxx and 5Wxx could be made with Group II (with a bit of Group II+ for 5Wxx). These oils are pretty good in terms of their stability and sludge performance but tend to have high Noack (especially the 5Wxx oils). You can get full and semi synthetic 5Wxx oils and their primary advantage is lower Noack. They are a bit better than full Group II 5Wxx oils in terms of stability, etc but the performsnce gap is far, far smaller than in the days of Group I & Group III oils.
In terms of additives, things are very much of a muchness. Because if the way the specs are written, it's quite realistic to have one additive package at one single treat rate in all oils, be they full mineral, semi or full synthetic. Often there's a overlay of something extra for reasons of differentiation but the core can be fixed.