Part Horse Hockey...

Petroleum oil may not "Shrink" Seals any more than a Synthetic. It has to do with the additive packs. Seal Swell Agents are added to the oils and many companies use les than what is needed so the seals will actually harden and not be able to do their job.

I have seen where an oil leaker will actually quit leaking over a period of time after switching to AMSOIL. AMSOIL in cases, will clean out the sludge deposits built up in the seal and then soften the seal allowing it to work again.

But then in the case of certain Ester based synthetics, they will clean out the sludge and attack the seal where it will actually leak worse... Seal compatibility can be an issue with Ester Based Oils.

Doc