Make sure the PCV system hoses and fittings are clear before replaceing the rear seal. There's also a hose connection on the throttle body flame trap that often gets plugged and needs drilling out. You can try easing into a full synthetic oil engine by just adding one or two quarts to the first few oil changes and gradually increasing the concentration. But at your current mileage why spend the extra $$ for synthetic oil? It's really not going to benefit you very much. Just use Castrol and a good Mann filter.