hour meter has oil inside the glass window

why is there oil in the hour meter and how involved would it be to fix. I am buying a D4 tomorrow and was told the hour meter has oil inside the glass I'll be traveling 850mi to get to the machine and was told the dozer was ready for work, I would hate to get it home only to find out I need to rebuild the injector pump any insight's would be appreciated

Its just the oil seal thats bad on the hour meter housing. The shaft may be grooved also from wear. Easy to repair.

Thank I really don't know much about these machines but I was just watching a video and it a very easy fix if even necessary