Q: Why do I have to change my oil?
A: Oil becomes unusable for several reasons: Oxidation is the breakdown of the oil itself, and can lead to a lack of lubrication and viscosity changes. Dirt and sand particles can get into the oil, but that's mostly handled by filtration. Fuel contamination can cause viscosity changes and potential flammability problems. Coolant leaks, perhaps the most difficult to catch, can overtake the detergent and dispersant additive package in the oil quickly (depending on the amount of coolant leaking into the oil). At around 500ppm (parts per million), or 0.05%, of contamination, the oil becomes hazy. Coolant in the oil will also lead to the production of phosphoric acid from one of the main oil additives, ZDDP, which is corrosive and defeats the anti-wear additives.

Q: Are there different oils for different engines?
A: Absolutely. Several things determine whether an oil is right for a particular engine: the right viscosity, the right anti-wear package (additives) and it must be non-corrosive. Over the last 20 years, the chemistry has been constantly changing. In particular, the detergent package in oil for diesel engines has changed. In the past, a 1% soot content in oil in a diesel engine would have been considered bad. The acceptable percentage has now changed to 4-6%. Oils for diesel engines are being formulated to suspend more soot, since environmental regulations require reductions in the amount of soot in the diesel engine's exhaust. So exhaust gas recirculation is putting the soot into the oil, and not into the environment.

Q: My new Honda's digital odometer display tells me what percentage of the oil's life is left. How does it know?
A: Some manufacturers are now including oil change recommendation algorithms with the electronic displays in their cars (Honda, Chevrolet, etc.) that essentially calculate a useful life based on time, temperatures, rpm, mileage and perhaps other factors. These algorithms assume the use of recommended oils (or those of equal qualities), but do not take into consideration such factors as the use of non-recommended (cheaper or less suitable) oils or oil leakage that is replaced with new oil (so that the total oil condition within the car may be okay while the electronic algorithm is recommending replacement). In these systems there is no actual contact with the oil in the vehicle; the algorithm is simply making a guess based on laboratory data derived from tests with the recommended oils. These algorithms, then, cannot detect or take into account soot, coolant or fuel leaks, particulate inclusions or other sources of contamination.