Standard hydraulic oil should work if that's the problem (wrong oil). The challenge is not the viscosity ( ATF is much thinner than any HYD or engine oil you're going to find), but what the oil is designed to do. ATF is designed to clean, remove heat/moisture & grip ( Dextron,vs Ford, etc), whereas hyd & engine oil are designed to not only remove heat & such, but lubricate as well- there are no additives to promote engagement of any kind.