Personally, if you have a LS, I'd do it every 30k to 50k. Even more if you do severe towing or city driving. And don't forget your ~5 oz of friction modifier!

Non-LS guys out there... it really doesn't matter as much. I'd do a change interval of 50k to 75k.

Why more for the LS? The Ford factory LS seems not to last long.

I've even noticed a slight difference in mine over the years.

 

What is LS? Also do I need to add 5oz of this ''friction mod'' to both front and back when I change the fluid? Use synthetic rite?
Thanks

 

LS=limited slip, it's a clutch pack that's installed in the rear differential to help increase traction by allowing both rear wheels to receive equal power.

Synthetic all the way and you only need the modifier in the rear if yours has a LS.

Use 75w90 in the front and 75w140 in the rear.

 

Thanks TBIRD! I was sure needing to know the type, my next question actually...

How do I know if mine is LS? I bought it used and had to buy a owners manual for it...

 

Check the axle code on the drivers side door jamb.

If it says:
H9 – 3.55 LS
B6 – 3.73 LS

 

Thanks AGAIN Tbird!