Sonja uses 2 ounces of oil for 8 ounces of peanuts to make 2 + 8 = 10 ounces of peanut butter.
Her last batch weighed 20 ounces, so she made 20 / 10 = <<20/10=2>>2 times her ratio.
Thus, she used 2 * 2 = <<2*2=4>>4 ounces of oil.
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