75w-85 or 75w-140 diff oil when changing rear gears

[mohadnuj111]

I have a 2018 manual gt and I’m changing 3.55 to 4.09 gears and I can’t decide which oil to use.
Theres gear change instructions pdf by ford performance states that when changing rear gears to use 75w-140 oil with friction modifier but am not sure if this also applies to S550 mustang. The 18 gt user manual on the other hand states to use 75w-85 oil. Car is stock and I live in a fairly hot climate.

Anyone could clarify please.

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[Flyhalf]

What type of car "usage" ?
Steeet ? Drag? Roadcourse?

 

[mohadnuj111]

Street only.

 

[Flyhalf]

75w 85 absolutely
No need of thicker oil .

 

[Cory S]

I use 75-90 synthetic. No friction modifier. Been doing this for 25 years.

 

[K4fxd]

Due to the added heat from the steeper gear I'd go with what Ford Performance says. It is their gear set, correct?

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