I've changed all the family personal cars to AMSOIL synthetic over the past 25 years--most had 100K on them when acquired. My customers have changed 100s of cars with 100K or more, some over 200K.

I recommend using a good engine flush, such as made by AMSOIL or BG before changing to synthetic in cars with over 25,000 miles. However, if you don't, or won't, use a flush, then plan to change the oil filter after a couple thousand miles, and maybe drop the first batch of synthetic after 4-5,000.

No synthetic on the market today (or in the past 2o years) will attack any seal or gasket material. If a gasket or seal leaks after synthetic is put in, it was already bad.