I don’t think it’s thinner if the synthetic you are using matched required spec for you car. Maybe synth oil is more slippery. My daily driver is a 2008 Honda Civic with 110,000 miles that I always used synthetic Mobile 1 or Penzoil 5w20 and I have no leaks. My other vehicle is a 2000 Toyota Sienna with 90k miles on it and I’ve been suing synthetic since I bought it 50,000 miles ago and again I have no leaks.
I think that if there are no issues with you car synth won’t leak. If it does leaks, I think eventually you will see the leak later with convent oil.