Although it looks like auto trans fluid, it isn't and do not try using ATF, it won't work. K&N has a special oil, as does Amsoil. I have seen the K&N oil at auto parts stores like Auto Zone and Advance Auto.
Look for a K&N Filtercharger kit. This also includes a spray cleaner for your air filter as well as the oil. You can get it at most of the major auto supply places like O'Reillys ( my fave cuz the wife works there), Auto Zone, Advance etc.
If your filter is the foam rubber type, you can also get the spray on oil at your local motorcycle shop, as most handle UNI air filters which are the foam type.