Here is the seal that goes around the shaft on top of the gear box.
Unless you can see that the oil is coming from around that shaft seal I would not worry to much about a little oil.
I have never replaced one of them seals myself, but I know JW has. Maybe he can add something to the topic, but I believe you will have to open the gear box up to replace that seal.
Here are some photo of the gear box if you would like to see inside one. Direct Drive Gear Case - Transmission Photos
How old is the washer?