Engine flush...it's just a solvent that attacks slude from old oil. Lots of different brands available. I only use Mobil 1 in my vehicles and change it every 10K miles/ filter every 3K. Synthetic will not cause leaks but may reveal a worn seal by seeping past it. If this happens, it was time to replace the seal anyway. It was just gunked up with potentially abrasive sludge and the syn disolved it away.