The major difference between synthetic and petroleum derivative valve oils is the rate of evaporation and the residue left when the oil evaporates. Synthetics have a very slow evaporative rate and leave no residues on the valve piston. Petroleum derivative valve oils evaporate rapidly leaving a varnish like residue. This means that when using petro base valve oils it is nessecary to lube the valves often. To test various oils for evaporation and residue, use a sheet of glass that is mounted at a slight incline. Apply a drop or two of each oil to be tested near the top of the incline, which will show the viscosity,( thickness ), of the oils. Leave the test panel for a few days and you will observe that the synthetic oil has evaporated very little, if any. Discoloration will show the residue left by evaporation.