T3 oil pressure switch

I have just taken out the oil pressure switch which has a copper crush washer. My new one has a washer already on with a wavy profile to it. Will this flatten down to form a seal or should I take it off and use an aluminium washer I ordered with the switch? Any thoughts would be appreciated as I’m going to down spanner until the forum has stopped laughing and hopefully replied. Cheers Dougie.

i always use copper

Use the washer provided and save the ally washer for later.
Like Keith I prefer copper as road salt will attack aluminium washers (especially on brake lines).

Thanks for replies, I will do as suggested.