first of all epx 80/90 is not lsd oil, its only back axle oil , it would'nt damage your differential But you need lsd oil for your mechanical cusco lsd to work properly (mz or rz?)
I wouldn't say its the reason your hearing a whine tho,
Is the cwp in good condition ? is the backlash , cap spread and pinion bearing preload set up properly? or did you just buy a whole unit ie final drive and diff aready together and just fit it?

