Answer. Crude oil is the dark brownish viscous liquid formed by decomposition of remains of dead plants and animals in the absence of air because of high pressure, temperature and bacterial effects. When it is heated in a furnace up to 400°C under high pressure it gives important fraction of petroleum.
While, residual oil is the left behind residue of crude oil that does not vaporize when crude oil is seated in a furnace up to 400°C under high pressure. Residual oil is collected from the fractionating volume and is heated above 400°C to obtain four fractions; lubricants, paraffin wax, asphalt and petroleum coke.