I had the understanding that synthetic oil in a older engine caused tha rubber seals to leak. A lot of people believe that synthetic oil in a new engine will not stick to the sides of the cylinders because there are no score marks on the cylinder walls unless you run the engine with regular oil for at least until after the first oil change.
Oh I've heard all the theories. I work at a engine reman shop, not that it necessarily qualifies me for anything, but just that it puts me around this stuff all the time.
Generally, a "score" on a cylinder wall is a bad thing, like a scar. The typical theory is that the synthetic is "too slippery"... and that there's essentially not enough friction for new rings to properly seat on a freshly honed cylinder wall.
Some people used to say that when you switched to synthetic it swelled up your seals, and that if you switched back it would "shrink" them and then you'd develop a leak - or that the "molecules are so small that they'll just leak out everywhere"...
It's all mostly crap, but the molecules are smaller, and more uniform, and because of that, if there is an existing leak, or the motor does have say a valve stem seal that's letting some oil by, or rings that aren't sealing well, synthetic will sometime seep by faster. Even then, the term synthetic is kind of a gray area anyway...as evidenced again in an article posted by PGreensvt in this thread
switching to synthetic oil
which discuses the what could potentially be great oil, even if not derived from what is traditionally revered as synthetic base stock. talks about how castrol syntec changed formula to what's not technically synthetic and mobil sued them and how it all plays out. Great link from Mitch.
I generally think I understand the general point of synthetics, and what they're good at, and bad at... I'm just always on a quest to know why, or how something works the way it does. In this case, it's just very scientific and hard to fight through the marketing jibberish.
Any word on what base stock rotella t5 or t6 is derived from?