Basically an 80W-xx oil will be as thick when cold no matter what the "xx" number is. That is the hot viscosity, so an 80W-90 will be thinner when hot than an 80W-140 but that hot viscosity is still thinner than the cold viscosity. The 80W-140 will simply not thin out as much as the 80W-90 will as it heats up.

I fill my rear diff by backing up my driveway onto a set of ramps. This tilts the fill plug up higher and I just fill it until oil runs out like normal. Usually takes around 3 qts.

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