In-Store Item Transformation?

I have a requirement to be able to order multiple diesel fuels, (i.e. summer diesel and winter diesel) directly from the supplier to the store by separate item numbers and track the inventory by a single item (diesel). When the two separate items are received, they are both deposited into the same in-ground fuel tank at the store and mixed together. When the customer buys the fuel, they are really only buying 'diesel fuel' which is comprised of a blend of summer and winter diesel (ratio depends on how much of each fuel is in the tank).

Does anyone have any suggestions on how to achieve this in RMS and SIM v13?