I see a fair number of posts where people are wondering what gears they have or if they have posi. Rear ends take oil to run just like your engine. You wouldn't think of letting your engine run old oil so why is the rear end any different. The oil in the rear end can use up hp and fuel milage and the lack of oil can cause it to fail. If you don't know what is in the rear end then you don't know what the oil is like or even if it's at operating level. Who knows if the oil in the rear has ever been changed. You might want to change your rear end one day and it's always nicer doing it when you want to, not when you have to...It's easy to remove the cover to check what's inside and fill it up with new oil.