GTItoGT

Well, I've seen both used for gear installs, I bought one of the complete kits that comes from latemodelrestoration.com. It comes with 2 bottles of 75w90 royal purple and friction modifier as well. I used both and it filled the diff. right up to the top, not dripping but right there. worked out perfectly. But should I have used the 140? I've got no gear noise at all!!! I was worried about the gear whine that some people get but its dead silent. Perfect install by a good friend of mine.

jrockgts197

The only thing that would be affected is the viscosity at high temperatures. The 140 is recommended supposedly due to better protection at higher temperatures. I have friends that run both weights without issue in 05+ mustangs. If you run the car hard and the oil begins to break down you could develop some real problems, probably starting with the bearings. If it's bothering you then just change it and save yourself the worrying.