If we answered the question straight up, I mean, what's the point of that?  It's no fun at all that way.  Sigh.

So looking at the part's fiche (readily available on numerous web sites) it does show a washer being involved with the bolt in an intimate relationship.  The final drive drain requires one as well.  I think the coolant drain has one too.  Anything that has a fluid drain bolt generally has a washer involved.

Can this washer be purchased at any auto store or is it a specific one for the bike that you'll need to get from a dealer? My dealer has them for $1 but they're closed till Tuesday and I might want to change the oil today.

No magic in this washer. Any washer of correct size and intended for this use will work. Steel will not as it is harder than the mating surfaces and will not give the slight deformation required to make the seal. Use the old one. If it has ridged you can use fine emory cloth or sand paper to smooth the surfaces of the washer. It will be fine. Go ride.

These are real "crush washers" used on Nissan vehicles.  Some motorcycles also use similar washers for the oil drain plug:

I've been riding since 1965, I change oil every 2 to 3 thousand miles. Have never replaced one, and haven't had any leaks because of the washer being too flat

Looks like I'll pick up a couple, they are only a $1 a piece. Thanks all.

Looks like I'll pick up a couple, they are only a $1 a piece. Thanks all.

Whatever gives you peace of mind.

Everybody seems to think the purpose of the aluminum washer is to provide an oil seal. I always thought it was to protect the aluminum pan from being damaged by the steel drain bolt.

A steel washer would work fine on an aluminum pan as long as it was smooth and had no sharp surfaces to dig into the aluminum.

The crush washer is there to deform (crush) to any imperfections in the bolt head and / or bottom of the oil pan. Any moderately soft material will do although soft aluminum and copper are used most frequently. Aluminum is less expensive of course so that is why it is used almost everywhere as a crush washer other than on brake connections, which are almost always copper or soft brass.

Another thing that works well is a drain bolt with a groove for an O-ring in the face. The O-ring seals with minimal pressure and actually lasts a surprisingly long time but most people greatly over tighten them; the oil filler on the engine and final drive of the C-14 are good examples of both an O-ring sealed cap and a point where people greatly over tighten them.

Brian

Everybody seems to think the purpose of the aluminum washer is to provide an oil seal. I always thought it was to protect the aluminum pan from being damaged by the steel drain bolt.

What about one made of some kind of special wood?   

I don't think it would seal very well, I'm afraid.

I don't think it would seal very well, I'm afraid.

  Oh Sparky... you're losing it man. Where's Brian when I need him??

What about one made of some kind of special wood?   

Ebony wood?

Amboyan Burl!  There, I said it.  Are you happy?

Amboyan Burl!  There, I said it.  Are you happy?

There we go Sparky!! 

GM uses a special rubber washer (not flat) just under the head of the drain plug on many vehicles.  There is a recess machined under the head and that's where the washer fits.  It elastically deforms as you tighten the drain plug, and seals quite well.