Bruce Brown wrote:
The issue of synthetics came up, and our factory lubrication engineer flatly said, sometimes synthetics cause seals to shrink and create leaks. Not always, sometimes, and the older the piece the better the chance of leaks.
So yeah, it can happen.
Having said that, I've switched things over myself and never had an issue. In the OPs case it was an older unit and switching back cured it - I'm thinking we know why. :W
This has been my experience, also. Sometimes they leak or start burning oil, sometimes they don't.
You've fixed it, so sleep well!