I'll be changing the fluid in my 03 Mustang GT's rear diff next weekend and I'm having trouble finding the right weight. My manual says 80w90 or 75w140 and doesn't really specify a reason for going with one or the other. I've read a few posts on the various Mustang forums with differing opinions on which one to go with but I don't really trust anything oil related I read there. I realize either weight will likely work fine but was just wondering if anyone with an SN95 had any experience running both. The vehicle is an 03 5-speed GT convertible with 55,000 miles on it. It see's mostly very light use but (obviously) occasional fun.