Lower unit fill plug washers

[Gilera]

The genuine washers that go behind the fill plugs seem very expensive. My local shop charges $5 Australian. Thus seems quite ridiculous. Is there anything special about them? They are usually dark grey/brown or black fibre washer depending on brand, or sometimes like a nylon ring. What do people use as a cheaper alternative? Do these red fibre washers stand up to water/oil ok?

[4Speed]

Not sure about $ exchange rates but I purchased about a dozen Mercury gasket/washers from Ebay for less than $20.00 US. I searched "Mercury drain plug gaskets".

[AlliStan]

Put a couple in a cup of water, and a cup of oil, and let them sit a few days. You'll be able to tell.