Changing gears, what oil should i use?

[4.6snake]

ive heard a couple different things on what oil to put in the rear end. some people say 75w90 and others say 75w140. which should i go with? also, should i get synthetic? ive also heard about getting the friction modifier, but unsure of how much to use. thanks guys!!!

 

[jpajkos]

Get 2 bottles of the Royal Purple 75w 90. The friction modifyer is already in the bottle. Good stuff. You will need 2 bottles.

 

[Lightning Struck]

Get 2 bottles of the Royal Purple 75w 90. The friction modifyer is already in the bottle. Good stuff. You will need 2 bottles.

royal purple is good, dont red line make gear oil too?