This implies the problem could be fixed with the correct seal material..

http://www.carbibles.com/engineoil_bible.html

-Gaskets and seals! With the makeup of synthetic oils being different from mineral oils, mineral-oil-soaked gaskets and seals have been known to leak when exposed to synthetic oils. Perhaps not that common an occurrence, but worth bearing in mind nevertheless.

There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if you had used one product or the other for a long period of time. People experienced problems with seals leaking and high oil consumption but changes in additive chemistry and seal material have taken care of those issues. And that's an important caveat. New seal technology is great, but if you're still driving around in a car from the 80's with its original seals, then this argument becomes a bit of a moot point - your seals are still going to be subject to the old leakage problems no matter what newfangled additives the oil companies are putting in their products.

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The lubricity additives could also be very different between Pennzoil (which is largely mineral oil based) and Mobil 1 (fully synthetic). These are the "slippery" molecules, that could work their way through small crevices easier. Unfortunately, the exact lubricant chemistry is proprietary to these companies. So finding out what ingredient it is in the oil, exactly, causing the problem would be tough, I suspect.