Not all crude oils are suitable for crude oil washing. Some very heavy or waxy crudes because of their viscous or waxy nature could increase rather than reduce the clingage. For a crude oil to be suitable for crude oil washing its viscosity should be less than 600 centistokes at discharge temperature and this temperature should exceed its pourpoint by at least 100 celsius. The division between crude oils and products and between clean and dirty products is not as clear cut as perhaps might be expected. Crude oils may not always be shipped in the same state as they are produced from the wellhead. They may be treated by the addition to or the removal from them of some components. A cargo described as a feedstock may be a product, such as dirty naphtha, or a topped crude oil. The dividing line between a dirty (or black) product and a clean (or white) product is blurred. Gasoil falls on the dividing line. A light coloured gasoil will almost certainly be a clean cargo whereas a darker gasoil may be regarded as a dirty product and shipped on a blackoil carrier. A product carrier with good segregation can carry both clean and dirty products. Bitumen is a black product which is solid at ambient temperature but not a lot of bitumen is now shipped in bulk and most bitumens require a specially designed ship with very high heating capacity. Bitumen cutbacks (bitumen to which a solvent has been added) need less heating. Bitumen will foam if any water is left in the tanks and special precautions are necessary. Other black products include the very heavy fueloils (a few of these may have a specific gravity as high as 1.00 and carbon black feedstock may have a specific gravity of more than 1.00), waxy distillates, diesel oils and dirty gasoils. White products include clear gasoils, lubricating oils, kerosene, aviation turbine fuel, motor gasoline and aviation gasoline.