The bolts and nuts are usually the cheapest galvanized steel bolts that the hardware store has.

The bolt diameter is usually twice to four times the flange thickness.

Bolts are usually tightened so the sheet gasket is compressed 10% to 40%.

Sealing is only a minor part of tank design. Wall thickness is probably the major part of tank design/cost. Gasket material and installation are also a major part of the design.

If you would explain why you are interested in "oil leak in transformer", we might be able to provide more specific details.