Synthetic oil is a bit more expensive to purchase, but it does have the advantage of longer protective life (i.e. more miles between oil changes, 15,000 vs. 5000).
The molecules are a bit smaller, therefore it does have the characteristic of being able to seep past aging seals and gaskets that would not normally have leaked with dino oil.
My point is this...if you change your oil at regular intervals (5000 miles or less), then stick with dino oil. Otherwise, there is no harm in using synthetic on an older motor. The potential for new leaks cropping up varies from one vehicle to the next.
Newer dino formulations have been upgraded over the years, and are safe to use at up to 7500 miles in many cases.
I use dino on all of my cars except the ML. It has the FSS intervals set at around 15K, so I started the maintenance program with synthetic oil right from the start.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car