Lucky!

[Conrad]

Boy did i dodge a bullet! Changed the oil in my bike, and after putting the plugs and filter back in and filling with oil, i find this little guy stuck to the old filter yet! Luckily i hadn't started it yet, so no oil in the filter canister yet
Just glad it didn't stick on the filter when i threw it out, i never would have known!

[GrandpaDenny]

LOL I've done that. Forgotten all kinds of things at various times. Used to be it took me three tries to actually get an oil change done. Expensive way to do things! Last couple of times I've finally got it down to a science - didn't forget anything, didn't put anything in upside-down.

[SgtSlag]

That is the washer that sits between the filter's central, rubber O-ring gasket, the bike's filter spring, with the filter bolt going through all of them, holding everything in place. It prevents the filter spring from digging into the filter's center rubber O-ring gasket assembly. I left mine out for a couple of years, on my Honda (see below). The filter bolt has the bypass valve inside of it (Honda and Kawi seem to have used the same filter bolt, just different lengths!). I believe that if the filter becomes blocked, oil pressure will compress the spring, pushing the filter down far enough for oil to bypass it, keeping the oil flowing through the engine. That is likely the only time that washer really comes into play. Chances of the oil filter plugging, are minute, as long as you change oil and filter, on schedule (I always change the filter with the oil, always). Oil, and filter, are dirt cheap, in the long run.

My '79 Honda had an identical washer and spring setup. I left the same washer out of that bike, two seasons, before I realized it should be there. Saw it on a schematic drawing on Bike Bandit's parts website: ordered a replacement, and added it back into the assembly when I got it. No harm, no foul, no damage. The XII and my CB750K used the same identical oil filter, and steel filter washer, as well. Cheers!