I have a 2003 Ford Explorer, 4wd, 4.6 cu in, 5 spd auto. I'm looking to replace the rear differential oil. The Owner's manual specifies 75W-140 synthetic, however the differential case casting has 75W90 cast into it. The tag bolted to the differential has a series of numbers and an L, so I'm assuming it's a limited slip. Any suggestions as to which oil is correct?