Synthetic oil doesn't cause leaks if you use it in a low mileage vehicle and use it consistantly. Many people are able to switch over even in high mileage cars without delevoping leaks. Sometimes leaks will appear if you switch to synthetic in a high mileage vehicle. The reason for this, I am told, is that in high mileage vehicles that have always used regular oil, a thin layer of varnish can form over gaskets and seal over many years. When you switch to synthetic (which does not do this) it will eventually clean this thin layer of varnish off of the seals and gaskets. Given the age of a car, it may not be surpirsing that gaskets or seals have dried or shrunk or cracked over the years. The only thing keeping leaks from happening in this case is the thin layer of varnish caused by the regular oil. When the synthetic cleans this off the leaks start to show. So, synthetic doesn't cause oil leaks, but it can clean up your engine so that leaks you might otherwise have had will appear. You said you ML is a 98. I wouldn't think that switching over would be a problem for you. Personally, my car has run, and run well on regular oil for 180,000 miles. I don't run at excessive engine temperature, and I like to change my oil frequently so synthetic doesn't really appeal to me. I can't see the benefits in my situation. If you want to switch I don't see any problems.
1995 E420 - retired
1986 420SEL - retired