Join Date: Sep 2013
Location: Nova Scotia, Canada
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The seal has been leaking for quite a while as you said. It probably just had a little oil sludge built up around it. When you did a oil change you put in a good quality oil with lots of detergents. This oil probably just cleaned up the contaminates and caused the leak to happen faster.
Or sometimes axle seals just start leaking quickly. I have had that happen without doing an oil change. Could have been a stick, rapped with grass, etc...and the seal got damaged on your last ride. It happens sometimes.
I just changed my front diff pinion seal today because it started leaking on my last ride. Didn't leak a drop before it, but was leaking lots by the end of it, and had let water in as well, milking the oil.
Additionally, it is a known fact that some oils will leak past worn seals faster than others. Many synthetic oils will do this due to its superior lubricity.
Either way you have a worn, leaking seal. Replace it and ride on.