They're just so easy to lose if you don't keep them somewhere  

The reason the copper washer did not fit is that you have an oversized plug like this one, intended to repair stripped cases:

//minimania.com/part/DP2/Drain-Plug-With-Self-Tapping-Threads-Mini--Mini-Cooper

I routinely found those larger fiber washers cobbled onto oversize plugs.  I do like the 'captive' o-ring style, and in fact in the later 80s, you'll find them on the oil pipe banjo bolt.... they work fine on the standard size drain plug!

My mistake Robin.   

Ian, the fiber washers are fine also.  None of the alternate sealing washers are expensive or hard to find. 

Don't know if the suggestd solutions are difficult to find/expensive... However, simple generic fibre washers are a cheap one shot solution. And I haven't stripped  a drain plug in around 40 years.  KISS

Cheers, Ian

Sorry for the confusication Doug. Both you and I have recommended the steel/rubber washers for a while. Does a fine job and avoids the over-tightening that strips the threads on the box. (bad day when that happens)
The reference to Chuck was about the search feature....

I didn't realize Chuck had posted that information.  I usually suggest the bonded metal/rubber seal.  You are looking for Dorman/Motormite 65629.  They sometimes have these out on the aisle of the parts store, sometimes they have them in back on the shelves behind the counter.

Thanks.  I appreciate all your responses. Turns out the copper washers I bought on this site do not fit the drain plug of my 998cc.  Went to the store, and got the right size. 

Whatever you use is preference. Yes, you can discard the old cork part. I personally don't like any cork gaskets. As they get saturated and old they tend to just fall apart. I use the copper crush washer that deforms to the imperfections of the two mating surfaces. If I ever find a suitable rubber gasket that is the perfect balance of strong, durable, and sealing, I'll promptly switch to it. 

what's important is that you get a sealing washer that you don't have to overtighten to seal the plug!

Thanks.  I take this to mean that I shouldn't or don't need to reuse the washers I found already on the plug. And, as I mentioned, I have purchased the copper washers sold on this site, so I will use one of those.  It sounds like the cork/particle-board washers just serve the same function, rather than providing any additional benefit.   

I'm going to channel Cheleker here....
From another thread easily found by a simple search of the forum:

Walk into your local parts store. They usually will have some generic oil drain washers in the "help" section. What you are looking for is a steel washer with kind of an o-ring molded in the hole. A 14mm will screw right on to the threads of your drain plug. I re-use mine for 3-4 oil changes. Shouldn't cost more than a couple of bucks.

Edited to add: Using a washer like this means you don't have to go "gorrilla" on the drain plug to get a seal... less chance of stripping the threads.

Another day, another question as I try to sort out the mysteries of my new 1988 mini (998cc).  I am doing an oil change and found these three washers (seem to be made out of some kind of particle-board material) on the oil drain plug.  Can I safely discard these in favor of the copper washer I just got from our host?  Thanks!