Group 3 is not a true synthetic base oil, it is mineral based. However, companies are legally allowed to market synthetics containing group 3 as "fully synthetic" because they are now refined enough to have similar viscosity index properties as a real synthetic group4/5. It is deceptive...
All I see is more marketing blah blah.. the "boosters" they are talking about are most likely added group 5 base oil since it is great at cleaning deposits, swelling seals, lubricity, film strength etc.
Yes. To what extent depends on the amount of PAO and esters used in the blend. The group 5 base oils are the main contributor to the seal swelling of HM oils. Group 5 is also great for cleaning engines.
There isn't necessarily some "seal conditioning" additive in the oil. The seal conditioning/swelling that prevents leaks is coming from the group 5 base oils (esters, AN, etc) inherent seal swelling properties, which are blended into oils to varying degrees to counter the PAO inherent seal...