The aforementioned oil not in Mobil 1 site for rear diff. Just for front diff and transfer case. Site recommended a non synthetic 80w/90 oil for the rear diff but not available. Any recommendations or experiences using 75w/90 on rear differential? I can change to Amsoil, appears lots of options for Amsoil. Experiences on either? Btw I changed the trans fluid to Mobil 1 synthetic and shifts noticeably smoother. Thanks.