Are you using a one side seal bearing or a 2 side seal?
Most engine bearings are only sealed at one side so you can try a 2-side sealed bearing.

But normally the construction with the groove in the crankcase and crankshaft should build up an hydrolic seal. Problem could be that today's fuels with less oils and also thin oils make the working of the hydrolic seal much worse. In the past we were used to run metal sealded bearings which are open.