What I had always heard is that Jurassic oils cause seals to swell
more than the synthetics do. Therefore, when putting synthetic oil in an
engine that has many hours on it using Jurassic oil, the seals will shrink a
bit and may cause leaks were there were none before.

What I have experienced matches this theory, as switching a 10-yr old car from
Jurassic to synthetic showed no ill effects - but existing leaks did get
slightly worse.