Rear dif oil.

[viper]

my washer looks siver in colour so dont think its copper, ive changed my final drive oil 3 times and not changed the washer and its never leaked , you could always just use a thin amount  of sealant if you are worried.

[Bryn]

My washer has been re-used and is not leaking... but I've bought a selection box of copper washers from ebay for next time... always need stuff like that in your man ( or woman) -den