When changing the rear differential oil,

1. Do I need to change the washer on both the fill plug and drain plug or just the fill plug?

2. Is this washer available at Autozone, Kragen, Pep Boys or is it only available at the dealer?

3. Also, for the power steering fluid, can I use any popular brand power steering fluid or do I have to use MB steering fluid or Mobil ATF D as specified in my manual?

TIA,

R.V.
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'92 300 TE Wagon (m103) 114K miles