I know this sounds a little unorthodox, but IMHO for better and / or faster results, soak the oil seal in hot oil for improved penetration of oil into the leather parts of the seal. I myself heat the oil in a tin can over a camping stove with the oil seal, until it starts to boil, then after leaving it to cool its ready to use.
This approach allows you to use a thicker oil if you wish, that is likely to stop in the leather for longer when its on the engine.