I would go with Synthetic since this is an item that does not generally leak and is a relatively high change interval. I went with Synthetic Mobil1 85W90 on both mine and the rear diff is quiet, no noise.
Nothing much to watch for unless the fluid hasn't been changed for 150k miles or has leaked out....just make sure to drain into a clean catch and check for any metal in the fluid.

Undo the top one first, then bottom. Make sure to 'crack' the bolts not tug on them as you can strip them...not something you want. Drain and run through some of the new fluid to semi-flush any leftovers out until it comes out clear. Replace bottom drain plug and fill until it starts to come out of the top then put top back on...done!

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k