According to NFPA 31, pipe connections that will permit gravity flow so the oil will be at the same level in both tanks at the same time between 2 – 300 gallon above ground free standing fuel oil tanks:

According to NFPA 31, pipe connections that will permit gravity flow so the oil will be at the same level in both tanks at the same time between 2 – 300 gallon above ground free standing fuel oil tanks: