Before switching an old engine to synthetic, a good cleaning is a good idea.
Check out auto rx ( www.auto-rx.com ) the stuff is costy, but it really works. I've been using it on my own cars and my customers cars for awhile now with great results every time ( I have no affiliation with them, their stuff just plain does what it says it does. I sell and install amsoil products out of my shop, but this is the cleaner that I use)I've averted a number of seal repairs with that stuff..