Rear differential LS additive?

[Afsigma21]

I am getting ready to change my differential fluids, I do have the electronic locking rear diff.

Do I HAVE to use the Motorcraft 75w85? I am sure I can use 75w90GL5 without issues.

Do I need to add an extra bottle of friction modifier like: https://www.amazon.com/dp/B00J0G09JW?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1

Or just a fluid like this with the additives already in it?: https://shop.valvolineglobal.com/pr...rzffOA14CE5cgnlFrZvlhGKBQTObZdfOuxT&Size=1+QT

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[RB205]

Lockers aren't limited slips. You don't need the additive but it also doesn't really hurt if you use the Valvoline with it already included.

Run the gear weight you want. Dana recommends 75w140 for these M220 Advantek axles. That's what I'm planning to run; Ford just recommends lighter oil for better mileage.