Why does a heritage softail have an oil tank ? – Cars & VehiclesReport
All Harley Davidson big twins use what is known as a “dry sump” oil system. That is, the oil is not stored within the crankcase of the engine itself. (As it is on most modern automobiles and trucks) It is pumped into the engine, cycled and returned to the exterior holding tank.
On Softails, the oil “tank” is located under the seat. On Dynas and Touring frames, the tank is under the bottom of the frame. Although the tanks are in different locations, they serve the same function and the lubrication design is otherwise the same.