An old thread I know, but at least I used search! :)

I'm about to do a rear diff fluid change on a 89 F250, 4x4, with diff tagged as 4L10 10 2 8H13 (4:10 limited Slip?)

I got 75-90 oil to put in it (it's just a farm truck, no synthetic required).
Just be certain, since I have limited slip I need to go back to 'the store' and get friction additive??