The Power Transfer Unit is sealed with no way to replace the fluid in it without complete removal and disassembly. The unit sits next to the Catalytic Converter, which generates a significant amount of heat. When my unit was disassembled, the fluid had the consistency of thick peanut butter, it should be the same as oil or transmission fluid. With the fluid so thick, the bearings do not receive adequate lubrication and fail. I realized I had a problem from the loud whining noise from the center of the vehicle while driving. This is a very poor design.