Switched our 98 Volvo S70 to Mobil 1 with over 90k on the clock. Now at 108k with no leaks whatsoever.

Synthetics dont cause leaks, they do a good job cleaning up and dispersing crud. If you had a leak before, the synthetic may cause it to leak a little more due its better detergants removing crud that may be filling the leak a little. Most synthetics have additives for the seals as well so I woudlnt worry about your seals leaking all of a sudden. You can switch whenever you want to, and you can also switch back whenever you want to as well.

P.S. There is a reason Slick 50 was fined heavily for false advertising and maing false claims. it doesnt work. Use a quart of Marvels Mystery Oil and it will clean up and quiet the valvetrain better than most additives out there.