What I meant was for him to go from dino to synthetic blend as a way of trying it out. Besides, synthetics are completely compatible with non-synthetics, so I don't see why not.

This whole synthetic in a high mileage vehicle revolves around the detergents that synthetic oils have. If there is a lot of grit & sludge, synthetic will clean it out and expose leaks that might have been "covered up." However, synthetics can also condition the rubber seals...there was an article on this.

All speculation aside, why not try it out for yourself? I switched to synthetic at 190K miles and have not had a problem. My buddy also switched at 140K miles and no problems either. The bottom line is that if your car has been abused (infrequent/long OCI's), you will see increased leaking. Just weigh the pros and cons and make an informed decision!