those high mileage oils have additives that condition seals that may shrink / harden over time. they are specifically aimed at preventing leaks. synthetics, when they first came out, had a problem with leaking because they can seep into the smallest cracks. most of them now have the additives to prevent that. if you are using the high mileage oil i would stick to it. i myself am using half "high mileage" and half regular dino oil because my main seal was slightly leaking (it has since stopped). i intend to switch over completely once i reach 75k. i had a bad experience with a leak i attribute to switching to synthetic oil and i would never switch a car over from dino ever again...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"