It depends on how you want the Oracle.
I like that it is unpredictable, that every outcome is always possible, that the modifier is a hope that things will get better, but not a certainty.
With the d6 a +1/-1 modifier eliminates the possibility of obtaining Yes and.../No and... as a result; in this case the possibility is reduced, but still possible (it lowers the probability of obtaining No and... from 16.67% to approximately 8.33%).
If you prefer to use a d6, you are welcome.