found this on the net. is this basically correct? YEARS ago when syntheitcs were introduced to the market there were seal swelling problems giving a bad rep to synthetics that perpetuates even today, simple chemistry changed this very quickly. There is a HUGE differences in one respect between conventional and fully syntheitc lubricants...WAX. In a conventional oil the molecules vary greatly in size and have a good pobability to sludge. As a gasket ages (due to heat and solvents) it dries and sludge build up prevents seal swelling components from reaching the seal (thus it cannot get revitalized). Cracks eventually result in an old and dry gasket...waxes and sludge CAN plug the small cracks preventing leaking. Adding a syntheic to a seal that has aged will, with syntetic oils improved solvency and detergency, dissolve sludge buildup. Combining the sovency and detergency with more consistant molecular size, leaking MAY occur. An engine that is well maintained with timely oil changes will tolerate a change to synthetics at any point is it's life.