Well it may or may not depending on the condition of your engine seals. The detergents in the synthetic oils will "clean-up" the varnish and sludge left by conventional oils. If your engine seals are worn, the synthetic will break down the oil varnish that may be maintaining the seal. So it's not that the synthetic oil caused a leak, it just that it revealed worn seals by cleaning the varnish off of them. Many people have switched to synthetic on 100,000+ mile engines with no leaks, so it just depends on how often you changed your oil and the overall condition of your engine.
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