Have you cleaned the Flame trap and checked the breather box. A clogged flame trap can cause excessives crankcase pressure and cause the seals to leak.

But beyond that.

Regular oil leaves deposits and sludge. that's why synthetic is better, no deposits and no sludge. However synthetic also disolves depostits and sludge left by regular oil. After 200K miles your engine was coated by a varnish. That includes the oil seals and the shafts that are turning inside them They formed a happily mated surface, micro groves matching micro groves, keeping the oil inside where it should be...glorious equilibrium.

when you started using synthetic you disrupted this age old (13year) equilibrium. The varnish began to be disolved, the engine cleaned...the spaces filled by the varnish opened...the oil she beginna to a leak.

What you thought was a good thing...using synth oil...turned out to be not so good.

Hopefull when you had the front seal changed you also got new cam and intermediate shaft seals and a new timing belt.