Go to AutoZone...they'll probably have some washers that are aluminum that will fit your oil cap.

If this will help, the size is exactly that of a quarter, but the hole in the middle is huge. Actually, just tried it and the hole is exactly the size so that a dime will fit through, but not a penny. These will be good references when you have to get a washer somewhere else (good note for me too if I ever run out and need one). AutoZone has a plethera of different washers (usually in a container behind the counter)...some made of plastic and fiberglass (like a gasket) and other materieals. If you can, stay with aluminum and AVOID COPPER. It will kill your threads on the aluminum oil pan.

I would not reuse an old aluminum washer if you can avoid it. It's soft but also already been crushed.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.