That is incorrect I have owned and worked on a half dozen of these
including my 22 forester I just changed the oil on a couple weeks ago.
the flat side goes towards the bolt.. rounded side towards that pan.
It actually crushes.
However, I doubt it actually matters. Should work fine both ways.. but it comes from the factory rounded side towards pan.
the first time you change it.. the factory one is painted to the drain plug.. you need a small screwdriver or pick to snag it.
every change after that it pops right off.
I highly prefer not to reuse because they crush and you can tell when its done crushing makes it easy to tell how tight to get drain bolt... however my first change on the 22 forester they sold me the wrong size crush washer.. so I had to reuse it... but I do have a stack of 6 correct ones now.
For those that dont know these arent solid flat "crush" washers like you see on hyundai etc.
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