The differential fluid bottle I got was thin and tall with a straight spout off the top. I put the spout in the hole and then could tip it up just enough to get the oil in there.

Important point - get the fill hole plug in the side of the differential loose before you take out the bottom drain hole plug. The side plug is difficult some times and you have to have it out to refill the diff, ergo it's done first "just in case".

I have a 14mm 1/2" drive socket that I put on my 1/2" impact gun and that loosens the plugs just fine.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered