I have a mule 3000 that i bought used. The carb cover has a small clear tube that has filled with oil. I took the cover off and there was oil inside the cover as well.[/align][/align]Does anyone know what causes this and what I need to do to fix it?[/align]

 

the tube i blieve is just a sight glass of sorts to monitor the liquid build-up in the airbox.. it should be normal evaporates from the crankcase that have accumulated over time.. if theres an excessive amout however, i'd start to wonder about the piston ring integrity and be watching ol consumption.. could be a clogged breather as well..
good luckm and welcome to KF!

 

Yup; I agree ^.