I just bought this "99" concord, it has 173,000 miles, has been using Pennsoil

What is your question?

This car has been well maintained, regular oil changes @every 3000 miles. Has always had 10-30 or 10-40 Pennsoil used in it. I am a fan of Castroil and would like to use it. Question is will changing to Castroil from Pennzoil cause any problems on this high milage engine? I have over hauled engines that have used Pennzoil, so I know that there is a lot of "varnish" and sludge. Will Castroil be a viable option? Also what about synthetic oil??