I would be a liar if I told you that switching over to synthetics could not possibly cause leaks around seals and gaskets. However, anyone who gives you the impression that this is the most likely outcome is either misinformed or lying themselves. Here is the whole story in easy to understand terms.
Back in the 70's and early 80's some synthetics were not blended correctly and caused engine oil leakage in some vehicles. Basically, the problem is that synthetic basestocks do not react the same way with seals and gaskets that petroleum oils do. PAO basestocks (the most common synthetic basestocks) tend to cause seal shrinkage. If the proper additives are not used, seals will shrink when using a PAO based oil, and leakage will occur.
Fortunately, oil manufacturers learned their lesson and reformulated their oil to contain the proper additive package which helps condition seals and gaskets to maintain their flexibility while also maintaining proper seal swell. In fact, the reformulation in most cases provides for better seal conditioning than most petroleum oils these days. Nevertheless, there is still a possibility of leakage if making the switch to synthetic - but only under certain conditions. Please allow me to elaborate a little bit.
Any of you who are considering a switch to synthetic oils probably know by now that petroleum oils do not necessarily keep your engine squeaky clean. Well, if you own an older vehicle (late 80's to very early 90's) it's possible that you have leaks in your engine already. Now, before you all string me up by my toe nails telling me there's never been a drop of oil that leaked from your engine, let me explain myself.
On older vehicles which have been lubricated with petroleum oils, seals and gaskets can begin to dry and crack. The reason you don't actually see leaks is because petroleum oils tend to burn off and leave sludge, grime and varnish on the inside of your engine. That's simply the nature of a petroleum oil's make-up. Now, conventional petroleum oils are not very discriminatory about where they leave those deposits. Therefore, some of the deposits end up around your seals and gaskets which actually plugs up the gaps which would have resulted in oil leaks.
"Well," you say, "I guess all of those synthetic oil nay-sayers were right. If petroleum oils keep my engine from leaking and synthetic oils might actually make it leak, I guess petroleum is the better oil."
Not even close. The fact is, it's a result of using petroleum oil instead of a high quality synthetic that resulted in dry and cracking seals & gaskets in the first place. As I mentioned earlier, synthetic oils now contain special additives which maintain proper seal swell and keep them flexible so that seals and gaskets don't dry and crack in the first place. So, for those people who use synthetic oils from the start (after a 3,000 to 5,000 mile break-in period), the problem never becomes an issue.
Of course, the next obvious question is, "If synthetics have those special additives, why might they cause leaks in an older engine? Why don't they correct the problem?"
Believe it or not, I've got an answer for that too. The "problem" is that there are also other additives that give a synthetic oil its detergency properties. In other words, there's other stuff in high quality synthetic oil which tends to clean out the sludge and deposits left behind by petroleum oils. Once these deposits are gone, the gaps around seals and gaskets become exposed and the oil might begin to leak in these areas.
However, there is good news. It's likely that the additives we discussed earlier will begin to lubricate the seals causing them to become more flexible and leading to seal swell which may plug those gaps over time. The only drawback is that there's no way of knowing how long it may take for this to occur or if the seals are already too far gone to be salvaged. If the problem doesn't correct itself, it is likely that the seals and gaskets would have to be replaced to prevent further oil leakage.
So, to wrap up. Petroleum oils can screw up your seals and gaskets and then fill the holes with gunk and deposits to cover their tracks. Synthetic oils come in and begin to clean up the place. Once it's clean, the holes might be exposed (if there were any) and the oil begins to leak. After a while (no way to know how long) the synthetic may be able to help the seals and gaskets regain their composure and stop the leaks
One very important point to keep in mind is that if 100 cars were put in front of you (all older vehicles lubricated with petroleum oil), the switch to synthetic would probably cause less than five or 10 of them to leak. The odds are definitely in your favor.
As is the case with most things, you tend to hear much more about the horror stories than you do about the success stories. I have spoken with numerous people who have made the switch successfully without ever having a leak. In fact, I personally have converted two older vehicles without even a hint of a problem.
Unfortunately, those who have had problems start screaming and yelling to everyone they know (with the best intentions) that synthetics will screw up your engine. Just take it with a grain of salt and you and your car will be fine.
NOTE: If you have a newer vehicle that has very few miles on it (say newer than 94 and fewer than 50,000 miles on it) and it begins to leak after switching from petroleum to synthetic oil, there may have been a manufacturer defect. Unless the manufacturer or dealer can prove that the synthetic oil caused the leakage, they are required, by law, to repair the vehicle under warranty (if you are still under warranty).