I think the main issue is (and i am not sure if this is still applicable or not) that Synthetic oil has/had detergents in it, so when used in a high mileage car it would end up cleaning up some of the sludge. When the sludge was cleaned up, it would cause leaks from old worn out seals that the sludge had basically kept from leaking.
Maybe Frosty is right and Detergents have been removed, idk.
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