Attention OriginalPoster &\or ForumAdmin:
consider changing title of this thread to "Differential GearOil Change"
Unsure about older F series trucks; for later smaller chassis Rangers\Explorers\Mountainneers\SportTracs\...
typical OwnersManual spec is 75\80w140.FullSyn in the Rear Diff & 75\80w90.FullSyn in the Front Diff,
FrictionModifier additive is needed if either is a LimitedSlip\TractionLok version.
Though here I've been using AmsOil# SAE250.FullSyn with AmsOil# SlipLok FrictionModifier with good results the last 2years,
for both the Rear LimitedSlipDiff & Front OpenSlipDiff on my 2001 SportTrac4WD,
for both the Rear OpenSlipDiff & Front PowerTransferUnit\PTU on my 2008 EdgeAWD.
My service routine is every 33kMiles\4~5years for the Diffs, 6~8kMiles\6mths for the PTU due to SevereHeatDuty.