It seems that the March '07 edition of Willie's Workbench stirred up some interest in oil-change intervals. This last month, I've had quite a few inquiries as to just how often one should change engine oil. The old standby answer was every 3,000 miles, but even that wasn't really a true answer. There are just too many variables to take into consideration, which include-but aren't limited to-time, air temperature, driving conditions, and engine load. One of the main reasons we actually change engine oil is to eliminate the contaminants that the oil filter can't catch. There is also the dilution caused by fuel and pollution from combustion chamber products bypassing the piston rings.
Almost every newer vehicle's owner's manual says that under "normal" driving conditions you can go 7,500 miles. But what is "normal"? This could mean an entirely different thing when defined by different people. Some vehicles even have a magic sensor that tells you when the oil is in need of changing. Going over double the old 3,000-mile figure is a great way to not only save some money, but to lessen the demand for more petroleum, plus it helps the ecology. But the question is, is it good for our engine?
Let's go back a minute to driving conditions. A hot, dirty, dusty environment or a hard-pulling loaded condition "uses" the oil harder than, say, a 65-mph freeway cruise. Therefore, under these conditions, you should change the oil more often. When I used to spend lots of time exploring Baja, it wasn't uncommon to change the oil after every trip, even if it was only about 1,000 miles. Short trips and stop-and-go driving can be almost as bad if the engine never gets warm enough to evaporate any moisture condensation that has accumulated, so it may require more frequent oil changes.
Generally, I go about 3,500 miles on a gas motor and 5,000 on my diesel for the oil-change interval. My choice of oil is Rotella T synthetic. Most likely, it's overkill by changing oil that often and/or using a synthetic. Makes me feel better, though.
Is Rotella the best oil out there? My research showed it's one of the better ones, and it's easily obtainable. I'm a believer in synthetics for their better cold and hot stability, and the ability to hold more dirt in suspension. The oil-change duration I came up with was after some research within the trucking industry. I made up some formulas that I used as a comparison-miles per quart capacity and miles per cubic inch of engine size-and factored in type of use to come up with my choice of drain intervals, which are definitely on the conservative side.
Some oil manufacturers, such as Amsoil's Series 3000 synthetic, claim up to 25,000-mile or one-year oil-change intervals, but they are careful to note that it is to be based on oil analysis, and just how often does the average person do that? Another popular brand used by a lot of performance enthusiasts is Red Line. They recommend 7,500 mile intervals for city and low-speed/low-frequency driving, and 15,000 to 18,000 for highway driving with a mid-mileage filter change.
As to oil analysis, is it a good thing? Most likely it is, in that it not only can catch a problem hopefully before any damage is done, such as excessive amounts of "dirt," or even a small amount of coolant in the oil. Is it cost-effective? Maybe, as a quality oil analysis will cost about $20 per test-not far off from what five quarts of oil and a filter will cost. Naturally, if you've got a 40- to 60-quart-capacity engine, the cost is a lot more justified.
Readers' response: A 200,000-mile gas engine is not uncommon anymore, what with modern precision machining and assembly. I would like to hear back from our readers as to their choices in lubricants, change intervals, driving conditions, and naturally, the number of miles they have on their engines. If I get enough responses back, I'll most likely publish the results. Send the information to email@example.com, or by mail to me at Four Wheeler, 6420 Wilshire Blvd., 11th floor, Los Angeles, CA 90048.