Have you ever tried to actually read your owner’s manual, especially the part about maintenance and service? As if this kind of stuff isn’t confusing enough, there are always two schedules listed, one for a vehicle driven under “normal” conditions and another for “severe.” But what exactly does this mean? I have yet to see an automaker that actually explains, in plain English, what these terms mean.

So this article is the first in a series of four that addresses the differences between severe and normal service recommendations. We’ll start with the most common type of service, the engine oil change, and in subsequent articles move through the rest of the vehicle.

Service Schedules for Oil Changes

Let’s start by trying to understand the differences between normal and severe. The severe schedule, which always has shorter recommended intervals between oil changes, applies if any of the following are true:

  • Most trips are less than 10 miles (16 km). This is particularly important when outside temperatures are below freezing.
  • Most trips include extensive idling (such as frequent driving in stop-and-go traffic).
  • The vehicle is frequently driven in dusty areas, like on dirt or gravel roads.
  • The vehicle is frequently used for towing a trailer or using a carrier on top, both of which place extra demands on the engine.
  • The vehicle is used for delivery service, police, taxi, or other commercial applications

If none of these conditions are applicable, you should go ahead and follow the normal schedule. Your owner’s manual will tell you the specific mileage suggestions for the engine in your vehicle, but make sure you have identified the correct engine, as many models have more than one. Keep in mind that the “rule of thumb” 3,000-mile oil change interval recommended by many oil change shops is an aggressive schedule that is likely to correspond with the car manufacturer’s “severe” category. It’s clearly in an oil change shop’s best financial interests to recommend frequent oil changes, but many manufacturers suggest 5,000-mile or even longer intervals under normal conditions.

There’s also a wildcard in the mix, which is that plenty of vehicles today have an engine oil life monitoring system. These work by monitoring the crankcase temperature, combustion chamber events (the work the engine does), and the time since the last oil change, and then using a computer algorithm to come up with an approximate percentage of usable life left in the oil. Under the right combination of events, these systems can recommend an oil change ahead of the service interval in your owner’s manual. Specifics for these systems vary from carmaker to carmaker, but overall, they are very accurate. If your oil life monitoring system says it’s time to change the oil, then you should do so regardless of mileage. Remember that the monitor needs to be reset every time you have an oil change, otherwise the system will be thrown off.

In The Real World

So let’s take a look at those severe schedule recommendations and try and figure out what they really mean. One of the biggest reasons why you’d use the severe schedule is if your daily commute is a short one, just a few miles across town, with plenty of stop signs and traffic lights between home and work. This sort of driving can be hard on oil because the engine isn’t allowed to completely warm up to operating temperature, especially during the winter in cold climate areas.

When the engine is cold, it operates in a mode that richens the fuel mixture, which causes excess fuel to spill down past the piston rings and into the crankcase. This dilutes the oil and breaks down its chemical fortifying packages, ultimately diminishing the oil’s ability to flow. This change in viscosity decreases the oil’s capacity to protect the internal engine parts, and to resist vaporization and oxidation. Other damaging effects are acid buildup and ash, causing further viscosity breakdown as well as internal sludging.

If you don’t understand what all these things are, suffice it to say that they’re bad for your engine and can lead to premature failure of internal engine parts. Trust me when I say you do not want to pay for an engine rebuild.

The other thing that really suggests you use the severe cycle is if your engine is driven hard. You’ve probably heard used car sellers say, “But all those miles are freeway miles,” to describe a high-mileage vehicle. There’s truth to the fact that highway mileage is easier on an engine, because it doesn’t take a lot of power to maintain 70 miles per hour on the highway.

Driving a vehicle that’s heavily loaded or towing a trailer -- or even just driving aggressively -- puts a much greater strain on the engine. When an engine is taxed from heavy loads, internal operating temperatures rise, causing evaporation and oxidation of the oil. This results in heavy “oil use” as well as caking and sludging. Oil use is different than oil burning. Instead of the oil entering the combustion chamber and burning, it evaporates through the breather system. The other condition, called oxidation, causes heavy internal sludging. Sludge is a heavy, oily, cakey substance that bakes onto the inside of the engine. Sludging starves the engine of lubrication because it soaks up the oil like a sponge as the oil passes over this unholy substance.

The other thing to be careful about is driving in a perpetually dirty environment. When an engine operates in an environment where fine dust and dirt is constantly fed into the air intake, this dirt eventually finds its way into the oil. This thickens the oil, which can become abrasive, causing internal wear. In addition, the dirt scrapes the sides of the cylinder walls, causing abrasion of the combustion chamber walls. If you live in the city and only rarely encounter an unpaved road, you shouldn’t worry about it when you do. But if you live in a rural area with plenty of dirt roads, the severe schedule is for you.

The 100,000-mile Interval

The following is an interesting story that illustrates how “normal” driving conditions affect engine oil longevity. About 12 years ago when GM officially launched its Oil Life Monitor, a caller to my radio show, America’s Car Show, asked me about the accuracy of the new system. My answer at the time was that I didn’t trust it. Shortly after the program ended, I received an e-mail from a concerned GM engineer who was on the team that developed the system’s algorithm. He said I didn’t know what I was talking about, which led to a conference call with the GM engineering team. They spent two hours enlightening me and I’ve been a believer ever since.

But what left a lasting impression was when they told me that they used an early ’80s Corvette for testing the new system, driving that car over 100,000 miles on the same oil without any serious engine damage. How could this be? By maintaining some simple operating parameters:

  • The car was primarily driven on the highway at highway speeds of 55 mph or higher.
  • The engine was run at least 20 miles each day.
  • The vehicle did not tow or carry heavy loads.
  • Crankcase temperature was kept consistently at 200 degrees Fahrenheit.
  • The vehicle was always operated in a clean environment.

While I wouldn’t suggest you try going 100,000 miles between oil changes, it’s clear that if you operate your car under the right circumstances, there’s no need to change your oil every 3,000 miles.