I just read in some other posts that switching a high milage car to synthetic oil from regular could cause leaks. Well, I bought my '94 850 wagon last year and I was sure it did not leak at the time. I have been using sythetic and over the past year it has been leaking worse and worse from all seals. Today it is in the shop getting new valve cover gasket and rear main seals. It never occurred to me that the oil could cause this, I just assumed it was time for them to give out. How is it possible for synthetic oil to do this - does it degrade the seals or something? Is it a chemical reaction with the rubber after being in contact with organic oil for years? And now that I have new seals should I go back to natural oil?

Confused oil changer :)

As DiFC points out, the answer is: No!

Rrom the mobil-1 website:

"Myth: Mobil 1 will leak out of the seals of older cars.

Mobil 1 does not cause leaks. In fact, new Mobil 1 with SuperSyn™ was tested in dozens of industry standard and OEM tests to prove its seal performance. It is fully compatible with the elastomeric materials from which all automotive seals and gaskets are made.

ExxonMobil engineers are wary of conventional oils that tout their use of additional seal-swelling agents. With extended use, these agents can over-soften engine seals, resulting in leaks. More to the point, an oil additive will not rejuvenate worn or damaged seals. The damaged seal may have been caused by a worn rotating metal component in the engine.

If an older engine is in good condition and does not have oil leaks, Mobil 1 with SuperSyn™ provides the same advantages as when used in a new engine. ExxonMobil recommends taking measures to repair the leaks, then using Mobil 1 with SuperSyn™. ExxonMobil also always recommends following the automobile manufacturer's manual for the proper oil to use. "

Synthetic oil is chemically inert, if anything the seals should last longer with synthetic. It's denied by Mobil-1 and Amsoil, but the reason that some people observe leaks in high mileage engines switched to synthetic oil is probably that the synthetic clears the crud deposited on the seals, and because if flows better than dino-juice at lower temperatures. Not only throughout the motor, but right past any marginal seals. I wouldn't hesitate to use dino-juice in an older motor that has been on a diet of synthetic, but I would be hesitant to begin using synthetic on an older motor raised on dino-juice. I have been experimenting by switching all my lawn and garden equipment to synthetic, no leaks so far, but obviously I'm risking less here than switching an old car.

-Punxsutawney Phil

No synthetic oil on the market today will attack any gasket or seal material.
Period!!!
If you have a false seal caused by a buildup of sludge or varnish, and the true seal has dried, changing to synthetic (or even another brand of oil) can cause the gunk to be cleaned away. Then you'll find the leak that was already there.
My 1985 Saab 900Turbo has a leaking valve cover gasket. Gee, must have been the synthetic oil which was put in originally about 180,000 miles ago (no, not the same batch of oil).

Your assumption that it was time is correct.