Differential Lube?????

What type and viscosity lube oil should I put in my differentials. I have a Corporate 14 bolt in the back with a detroit locker and a Dana 44 up front
with a ARB air locker. I would like to change out the oil, but don't know what kind to put in. I would like to use a synthetic, but I don't know what range or brand.
Thanks for the help
Brent 1983 K5 350
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You don't need a synth in the diffs. Any 75w90 or 80w90 gear oil will do the job. I've got a 14boltFF and Dana 60 front, both have Detroits, both get 75w90. If you do any water crossings, chances are good that theres water in the diff too, check it often, save your bearings and seals.

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