1 and 3L barrels are full, and then they are poured into 7L barrels three times. When the 7L barrel is full, there is 3× 3-7 = 2kg left in the 3L barrel.

2. Pour the oil in the 7L barrel back into the 10L barrel, and pour 2kg from the 3L barrel into the 7L barrel.

3. Fill a 3kg bottle and pour it into a 7L barrel, so there is 2+3 = 5kg in the 7L barrel, and the rest in the 10L barrel is 10-5 = 5kg, which is exactly divided into two parts.

The 1 step is reversed 3 times, the second step is reversed 2 times, and the third step is reversed 1 time, so it must be reversed at least 6 times.