I had this problem on my bikes a few years and here is what I learned. The impeller shaft may be grooved which would cause the seals to leak or wear out faster. When i replaced the shaft, my milky oil wnet away. Also, make sure the anti freeze you are using contains NO silicates, as they are abrasive.
I hope this is of some help. If you need part numbers for your bike, email me because I have a print out of the micro-fische.