You might still have a limited slip differential. What axle code do you have (door sticker)?

On my 2006, I use a gasket instead of the RTV.
Rear - Lube Locker LLR-F105
Front - Lube Locker LLR-D060

In the rear, I use Mobil 1 75W-140 and it already has the friction modifier in it. I do have a LS differential.

For the front, 80W90 is specified, but I use Mobil 1 75W90. On the bottle label, it states that it also has LS additive in it.

 

the additive/ modifier from ford is horrible smelling lol , but I suggest using their gear lube since it has no modifier and what the modifier does is let the clutches slip easier = less grab

so I avoid lube with the additive in them for the Ford rear and I use the OEM fill and then if it chatters and you dislike that you can add modifier at the cost of less LS action

the gut who sold it to you for the front is clueless but two bottles is usually sold for a rear fluid change IIRC

as far as the gear the front oil you bought being 5 w off I wouldn't sweat it
and like mentioned the front is an open diff so LS additive if already included in the gear oil won't help or hurt


dealers like the LS additive in the rear because it eliminates comebacks from customers who notice a chatter after a rear dif oil change, so they just dump it all in and kill the LS performance

 

My truck has no door sticker! I hate that!

I'll tear into it tomorrow. I bought some mobile 1 full synthetic but you make it sound like I should get the oem ford stuff instead..

 

After about 50k miles, OE LS is often not very active. I stopped using it at any post 50k changes in all my Ford vehicles over the years and never felt chatter.

If you are going to use a gasket rather than RTV, Mark's recommendations are the best to go with.

If you don't have a door sticker for whatever reason, if the tag is still on the differential cover it will show "LS" if it has limited slip.

 

After about 50k miles, OE LS is often not very active. I stopped using it at any post 50k changes in all my Ford vehicles over the years and never felt chatter.

Interesting. Thank you!

 

+1 on looking at the differential for the tag

 

To whom it may concern:

The front diff will need 4 qts of 80W-90. Where I got my spec from it stated it needed just under 3. That was not enough. Hopefully topping off the last 1/4 qt of dino gear oil won't sabotage the whole project.

As always, thank you guys.

 

to overfill the diff you have to be fast with the plug and like the smell of gear oil on your skin
or you can jack up the fill plug side of the axle ;-0

 

My 2006 owners manual states: The rear 10.5" stock differential capacity is 6.9 pints (3 1/2 quarts)