100k is hardly anything on a well built, modern engine. The idea that synthetic oil "causes" leaks or causes damage after using conventional for a while is really just a myth. Synthetic oil will potentially amplify a leak, but that's a leak that was already there and synthetic oil's flow characteristics just made it more noticeable. High mileage full synthetic oils contain seal conditioners that will actually work to keep your seals nice and supple to prevent leaks and slow existing seal leaks. They also have more anti wear additives than regular synthetic. Full synthetic oils are best in terms of cold start performance, flow characteristics, lubricity, and most of all longevity. Synthetic oils' viscosity does not degrade quickly like conventional oil. Overall it's a good idea to make the switch to a high mileage full synthetic. The Walmart/Costco branded stuff is super cheap, only a couple bucks more than their conventional. It's good oil too. 

If you were using high mileage, keep using it. If you used full synthetic, then continue using it. Moral of the story is, keep using what you used in the past to avoid potential oil leaks! Remember, clean and the right weight of oil is important.

Mobile one & Castrol has high mileage full synthetic oil

On my 2007 civic I put regular 5w-20 dinosaur oil in warm seasons and when winter sneaks in I put synthetic 5w-20 or 0w-20 clocking almost at 300k km had it for years and still runs strong and never had any problems.

I have an Infiniti m35 2006.  Ever since I got my car around 75 k miles I’ve used Mobil one high mileage. But recently a mechanic told me the full synthetic is the best. 

It really doesn't matter that much. All high mileage oils now are semi-synthetics anyway so changing to full synthetic isn't going to hurt anything but really I'd just stick to what you've been using if you aren't having any issues.