Synthetic Oil does NOT create leaks, in new or old engines. Synthetic oil flows more easily than the same viscousity conventional oil and the molecues are engineered to be polar, meaning they cling to metal and will have a greater capillary action. The result is that existing leaks in your motor may experience more oil passing thru the leak than it did with conventional oil.
That was not my experience when switching in a motor that 175k miles on it. Its more theory than practical experience, but a greater amount of leaking is possible.
SO, synthetic oil does NOT create leaks, it "MAY" make existing leaks a little a worse.
Some people will tell you that synthetic will wash away crud and dirt around seals and create new leaks. IMHO, that is very very unlikely. As well, some people believe Synthetic will break up existing crud and dirt in the motor and chunks may get caught up in the motor and cause damage. Again, IMHO, that is very very unlikely, fresh conventional oil can do the same thing, if the motor is that crudded or dirty, its going to happen wether you use synthetic or not.