paddy heffernan

a couple of year ago the diff oil leaked out of my diff when it was on ground during restoration work .when putting back in diff i decided to put in new oil .ford said the oil originaly specified was discontinued and gave me sae90.the diff is whiney since .can anyone tell me is this oil suitable for diff it seems very thick .i ordered new bearings and seals .when i removed diff and opened cover everything seems good and tight .should i just put back together and replace with different oil or go ahead and change bearings .