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Q: Does an index of 50 displayed on the oil service monitor imply half the lubricity of an index of 100? The index numbers get fairly low by the time the oil-change mileage gets much beyond 5,000. When might damage to my engine result? Please also discuss the differences among regular oil, the 50/50, and the full synthetic. I know you prefer the latter, but unlike me, it ain't cheap.

A: The oil service monitor displays the remaining percentage service life for the oil. It is a computer-based system that monitors the number of engine starts, RPM, coolant and oil temperature, throttle position, manifold pressure, etc. 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. Under consistent highway driving conditions the system will predict a significantly longer oil "life" than under more frequent startup/cooldowns, stop/go driving, acceleration, etc. And remember, the oil service life 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.

Q: I have a problem with both key fobs for my 2010 Jeep Grand Cherokee. I live on the west coast of Canada. We do have wet winters and my Jeep is parked outside. The key fobs take a heck of a lot of attempts to open the doors. I did replace the batteries. I took the vehicle to my local dealership and they said there was some corrosion in the fobs, which they cleaned. The problem has not improved and persists. Is there a system in the Jeep that should be looked at by the technicians?

A: Since the problem occurs when using either key fob, it would seem the fobs aren't the source of the problem. I searched my ALLDATA automotive database and found service bulletin #08-010-10 dated May 2010 which suggests that reprogramming the WIN — wireless ignition module — may correct this problem. This requires the use of Chrysler's wiTECH Diagnostic Application to identify the supplier of the WIN module (Marquart) and updating its software. This would be covered if the vehicle were still under warranty, but having had the dealer attempt a repair unsuccessfully you might want to ask for a customer goodwill adjustment. Never any harm in asking.

Q: On my 1993 Mercedes 300E, which has about 140,000 miles, oftentimes when I first start it in the morning the engine starts normally but when I press on the accelerator the engine has only a minor increase in RPM. It does not always happen. The problem seems to be worse in cooler weather. Occasionally the problem will occur after it is operating as it should when on the road. The engine just suddenly reverts to idle speed. When I start it and the problem is evident, I have to wait several minutes to a half-hour until it corrects itself. Do you have any idea what could be causing this problem?

A: By the intermittent nature of the problem, I'd focus on an electrical issue. A bit of research turned up several Mercedes service bulletins referring to similar driveability issues. A bad solder joint in the overvoltage protection relay module has been identified as a potential culprit. An updated relay module #000 540 52 45 is available as a replacement.

Other potential causes include a loss of signal from the speed sensor, a damaged protective screen in the mass airflow sensor or a faulty EGR switchover valve.

Paul Brand is the author of "How to Repair Your Car" and "How to Repair Your Truck and SUV," published by Motorbooks.