You can find that information directly on the label or metal plate on the tank.  In Canada, the thickness of the steel is expressed in millimetres, in the USA the steel thickness is expressed in gauge.
Depending on the year of fabrication, the approval it bears (UL for tanks sold in the USA and ULC for tanks sold in Canada), the type of your oil tank, single wall or double-bottom, the thickness of the steel used for the assembly of your tank can vary due to the different manufacturing codes requirements in effect over the years, that is normal, not all tanks were built to the same code requirements over the years.  There are no right or wrong steel thickness of steel, they are only variances.