But you need currency to buy oil. And the result is an "artificial" stronger dollar and a weaker euro - a different exchange rate - compared with if you could purchase oil in euro
So the real problem is that currency has a value all its own, entirely apart from what goods and services it can buy. As a result, there's no way to blindly equate X in one currency with Y in another, because the very act of exchanging one for the other affects their relative values. Transactions that should be purely economic in nature have unavoidable political undertones.