Do not switch to synthetic on a high milage engine without flushing first.
The synthetic can disloge chunks over time that are not disolved enough to pass to the filter( how do I know?..... out of sheer lazyness, I switched one of my own engines without flushing first..and I of all people should know better being an Amsoil dealer, after a few thousand miles the oil loosened up some carbon chunks on the bottom of the intake manifold and plugged up a hydralic lifter, so I had to pull the intake (chevy v8) and replace the lifter...less than fun in a van)

The Amsoil engine flush works great for most engines, and for severly grungy engines use Ol Schlamm Spulung made by Liqui-Moly , that stuff REALLY cleans ( I revived my diesel with it).