Use either washer but not both. Both will make a seal. The mating surface on the oil pan is quite flat and there isn't a problem making a leak proof seal. When I owned my Z's I never used a crush washer - just a plain washer and I never, ever had a leak. Don't overthink this. It's a simple mechanical seal.
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"America.... still builds rockets!"
Vette: 355 cid, 400 rwhp
'13 CTS-V Coupe: 6.2L LSA, 556HP
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