Fully synthetic engine oils have very strong, detergent properties, and if poured in those engines, which have been driven on comparatively thick engine oils, and semi synthetic or mineral engine oil, then seals leakages may start after some time.
May be, the leakages go away from the engine by itself, after driving 10-15,000 Kms, or 02-03 oil changes, may be not. In that case, you may have to replace the relevant seals.