15 cases
1) Consider that 1 gallon corresponds to 4 quarts. So we can write out:
30 gallon = 120 quarts
8-quart bottle per case
2) So we have in each case 8-quart bottles. Since we have 120 quarts then, let's divide it by the capacity of the case:
3) Hence, we'll have 15 full cases with those 30 gallons (120 quarts) bottles of oil.