An oil leak isn't usually a motor threatening problem unless you let it go unchecked. That being said some cars when switched over to synthetic from regular oil will start to burn oil or will develop more leaks. Running a regular oil is not going to damage anything, you just have to change it more often. I've seen cars that burn oil (blow by from the rings) with one brand and another brand (same weight, same type) the car won't burn oil, or it wont' burn as much. Often times when you switch the type of oil you're using you'll start to burn oil.