I am not so sure that it is just swelling the seals or more so treating them, not to dry out and become brittle. Fact- Over time, your gaskets & seals from pressure, heat,cold, and overall driving conditions will eventually dry out somehow. Additives like PAO, ester and god knows what exists in oils to treat these problems. Unless you can find a list of chemical compositions for each brand, it really is a shooting match.
When synthetic oils first came onto market, cars started leaking like a siv, cause they did not know the older seals would act in that manner. Syn oil manufactures had to put additives in the oil to stop older seals from drying out /shrinking and then...In new cars the composition changed in the seals, allowing for more long term flexibility with use of the syn oils.
Regardless of what oil I may use or blend myself, GF-5 rating is a good starting point.
Main GF-5 Site Testing
And when it comes to high mileage oil, many do not meet GF-5, mobil1 super 2000,high mileage does, and Pennzoil...check your bottles...before buying.