So how exactly does AeroShell 15-50 "promote" oil leaks? You make it sound as though it's attacking gaskets and seals in an effort to escape confinement.
I have Lycomings. One of the reasons I don't like Phillips oil is because I have to add the Lycoming additive separately. As you top off oil between oil changes it becomes a PITA to keep maintaining the right ratio of additive. I used to run Phillips in my last airplane, and frankly can't tell any difference.
For what it's worth, you do not need to run the Lycoming additive in all Lycoming engines. You can put whatever you want in the engines unless it is specified by AD to use the additive.
Camguard produced some documentation saying that the Aeroshell oil with the Lycoming additive in it attacked oil seals. I can't say that I've seen increased oil leaks on Aeroshell engines vs. Phillips engines though. It seems like they all leak.