Maybe you're a DIY-er and change your own oil, maybe you don't like to get your hands dirty but you're still curious about how your engine works, or maybe you just feel left out when car guys start talking numbers. Here's a basic guide to motor oil to get you started.
Introduction to Viscosity
The function of the motor oil is to get between every moving metal surface of your engine's components. A layer of oil between pistons and cylinders, shafts and bearings, cams and valves keeps things running smoothly and helps prevent wear and tear. The gap between some of these components is extremely small, so the viscosity of the oil you use can be a critical part of your engine's performance. Viscosity is a measurement of oil's ability to flow through a small opening at specific temperatures. Lower viscosity oils will flow more quickly than high viscosity oils, at a given temperature. SAE has developed a system for rating oil according to its viscosity at hot or cold temperatures. Most of the oil we find on the shelf at our local parts dealer will have a weight (same as viscosity for this discussion) designated by two numbers - 10W-30, for example. The first part (10W) tells you the viscosity at low temperature. A 5W oil will flow more easily at 0 degrees F than 10W. The second part (30) represents the high temperature (210 degrees F) viscosity. The numbers work the same here, too - 30 will flow more quickly than 40. Your engine will be required to operate over a wide range of temperatures, from cold starts in the winter, to boat hauling in the summer. These two-part oils were developed to protect the components at both extremes of the temperature range.
So what's best for your engine?
You should choose your oil based on your engine type and driving conditions. Always start with your owner's manual. It will provide recommendations from your vehicle's manufacturer. If you don't have that, you can still make a good guess. Think about the range of temperatures in which your engine will be running. Remember, you want your oil to flow easily through your engine. If you live in a cold climate, you can look for the lower numbers. If you only use the car when it's hot, or you have a high performance engine designed to run at higher RPM and temperature, go with the higher numbers. Castrol has provided a handy guide that should help most people choose the right oil. There are more choices, but chances are you'll find what you need there.
Time between oil changes
I've heard all kinds of advice on how long to wait to change your oil. Some suggest doing it every 1000 miles, others never change it. Some will say the oil companies recommend more frequent changes to encourage more sales. New oil and engines are expanding the maximum recommended gap between changes. For as long as I can remember the most popular rule of thumb has been to change your oil and filter every 3000 miles or every 3 months, assuming you use a conventional oil (more on conventional vs. synthetic oil in a bit). Fully synthetic oils can last 10,000 miles before they need to be changed. You can't hurt anything by changing your oil too much, but you will absolutely shorten it's life by changing it too little. Even the best oil eventually breaks down or burns off and becomes less effective at lubricating your engine. It's a safe bet to check your owner's manual for suggestions on this too, if you're not sure how long to wait. I use a synthetic blend in both of our cars, and try not to wait any more than about 5000 miles between changes. It's best to put on a new filter at every oil change too.