Consider lubrication engineers 1150 trans all for the automatics. I buy it in the 5 gallon pails for my automatic trans cars. A very large transmission cooler will make more of a difference than any oil you choose here. Red line MTL for my sons 5 speed focus. Tried red line 75-140 in the mitchel under drive in my f-250. Straight lucas pure synthetic oil stabilizer is much better. Less noise and smoother shifts. I use tractor hydraulic and lucas synthetic 50% each in the f-250 5 speed instead of ATF. Its better in every respect than atf except in very cold weather,and even then it clears up in less than 5 mile of driving. The 5-40 is a little thick for your mustangs. 0-20 or 5-20 redline would be better there. I tried 5-40 delvac in my Lincoln when I was buying it in bulk for my freightliner. Its a little heavy for this engine. That only gets worse if the lucas synthetic additive is used with it. The ford modular engine likes thin oil PERIOD. The small block chevy would probobly be happy with the used oil I take out of my freghtliner after 30000 miles. But if you want to spoil it like a grand child 5- 40 redline will be over kill enough to make you happy.