I've got a slight leak at the bottom of the cover so I'm going to change oil while making the fix.

My 1997 F150 XLT has a 8.8" 3.55 LS and the tag on the rear says 75W140 SYNTHETIC ONLY. Owners manual shows a capacity of 5.5 pts.
Nothing in mentioned in the service manuals regarding adding friction modifier.

Called my local Ford Service friend and he had trouble finding info too and was surprised he couldn't find anything. After searching some other sources he found 4oz of friction modifier is to be added.

Searching online you will find some folks saying no friction modifier is required with synthetic oil while others say to add 4 or 8 oz of FM.

I'm sure this was discussed here before at some point but thought I'd share what I learned in case someone else comes across the same issue.

 

Add 4 oz of the Ford XL-3 FM regardless of what fluid is used. Many complaints of people using even high quality fluids and still having issues till they add the FM.

First time I changed my fluid I used some Lucas 75w140 fluid and the XL-3 with no problems.
I had to change the fluid again due to an unrelated issue and used Supertech 75w-140 and the XL-3 and no problems so far (been about 1000 miles).

 

I normally buy the 7 oz tube of modifier at Auto Zone for less than $10. add the whole tube-- works great with any synthetic.

 

I had considered that but the auto store I went to showed they had 2 tubes in stock but they couldn't find them. . I ordered the gear oil online from them, website said they had 3 qts in stock but when I got there, they said they had to run out and get some since they didn't have any.

Stopped at the local dealership and picked up a 4 oz container for less than $7.

5.5pts filled it up just to slightly below the fill hole.

 

monitor to see if you get any rear end chatter when taking off from a stop light and turning a corner at the same time......... if you notice anything, add another bottle.

 

Will do. There were no issues other than a slight leak before I change the oil so hopefully there are none now too!