Mine had a small leak for about a year... Replaced it and it's leaking again now, been leaking now for about 4 months. As long as its a slow leak your okay.
You just don't want any of it getting on that back tire.
The extra weight from the oil/dirt on the fan might burn the motor up over a long period.
Come to think of it when I replaced the rocker box gaskets, I put a new fan on as it was making noises.
I have no plans on replacing my rocker box gaskets anytime soon unless the leak gets worse. But as is I don't see any drops dripping from anything. We shall see if the fan starts making noises.
It only took 2 years to start leaking again... Pretty ****** gaskets!