So you have the tapered roller bearings and the external seal. Sometimes you can wad the seal up when installing the axle. There is a little rubber lip that goes around the outside of the seal that can get damaged when installing the axle. Is it leaking around the outside of the seal or around the axle?
I used synthetic oil in hundreds of rear ends with no problems. Most new synthetic oils have additives in them that cause seal swell. Lack of seal swell is what caused oil leaks with the old origianl synthetic oils.