QUESTION: Does a reading of 50 on my car’s oil service monitor imply the oil’s lubricity has been reduced by half? This number gets fairly low by the time the car is ready for its next oil change. At what point am I risking damage to the engine? Also, can you discuss the differences between regular oil, synthetic oil and regular-synthetic mixes? I know you prefer the synthetic stuff, but unlike me, it ain’t cheap.

ANSWER: The oil service monitor displays the remaining percentage of service life for the oil.

The figure is generated by a system that monitors, among other things, the number of engine starts, rpm, coolant and oil temperature, throttle position and manifold pressure.

It computes how hard the oil has been worked and predicts when it’s time to change it. Engine damage is unlikely to occur unless the oil is not changed until long after the oil service system suggests doing so.

The system will predict a significantly longer oil life if you accumulate most of your miles under consistent highway driving conditions as opposed to doing a lot of starting and stopping.

And remember, the oil service monitor does not know what type of oil is in the engine.

Regular lubricants are refined from organic crude oil containing random-sized molecules. After refinement, a regular 5W-30 oil would have an average molecular size of a 5W oil.

Synthetic lubricants are refined, distilled, purified and chemically rebuilt for total uniformity and molecular size to provide improved lubrication characteristics.

All of the molecules in synthetic 5W-30 lubricant are identical. Semi-synthetic oils are just that — typically a blend of 20 percent synthetic and 80 percent organic lubricants.

Paul Brand, author of “How to Repair Your Car,” is an automotive troubleshooter, driving instructor and former race-car driver.