- Nov 24, 2003
- Ellicott City, MD
I have a 2006 Mustang GT. Why does the V8 engine specify 5W20 yet the old-style 8.5" rear end specify 80W140 (not 75W90). I would argue that the rear gear has just as much potentual, or even more, to gain mileage with a lighter oil. Ford knows that the rear gear needs the heavier oil to be reliable (they've fixed alot using 75W90)- wonder if they also know that for the engine?