The full synthetics that I've used (as opposed to oil calling themselves synthetics, but actually containing largely hydrocracked oil) will quickly clean out sludge that oils using high quantities of viscosity improvers will often leave behind. If those deposits are plugging holes, then the engine COULD develop leaks. If the leaks are due to gasket or seals shirking however, Mobil synthetics will usually seal the leaks by causing the seal to swell slightly. Sometimes the leak will appear (as the sludge is cleaned out, but the seal doesn't swell quite as fast as the sludge disappears) but if you wait a few thousand miles, the seal WILL swell shut. I have NEVER had an engine leak only because I changed it to synthetic oil and I've change over diesel engines with more than 200kmi on them from conventional oil to synthetic oil. Such leaks WERE common with some early synthetics.

Marshall