Differential oil additive or not?

[chiggerranch69]

My 69 has the factory "locker" as the tag implies, but it appears to have clutches inside like L/S (trac-loc?). I know it's factory. Question is, did Ford ever require friction modifier, or am I alright using normal gear oil? When I pulled it apart it looked like a 37k mile gearset should, except for the oil turning back to dino dung from sitting 30 years!

[FORDification]

Yes, if it's got clutches you'll need to use a friction modifier. The only limited-slip I'm aware of that doesn't need it is the Eaton TrueTrac, because it's a gear-driven setup.

[chiggerranch69]

That's what I thought. So when Ford and Dana call it a locker, are they refering to the trac-loc? Or am I just seeing the mecanism of a true locker? Hard to see in there with the side gears in the way.

[Hawkrod]

Ford did not use a "Locker". The rear in a F100 is a Limited Slip and requires additive. The only Lockers Ford used back then were in some limited production specialty performance cars like Boss and SCJ Mustangs/Torinos, Shelby's as well as the Merc equivelents. The tag only implies it has a limited slip (that is what the L means in the ratio ID code). Except for those listed above, Ford did not call them lockers in any literature or on the tag so I am not sure where you got the idea that it was a "Locker". Hawkrod

[chiggerranch69]

I know it isn't a locker, hence the ref to clutches. I got the idea from the Dana diff guide on this website that has a column for L or NL, impling locker.

[big block]

diffently need to add friction mod. even though most of the gear oil containers say it is already there. add it any way.