Chances are you’ve read a lot about the extreme demands that 2002 diesels will place on engine oils. They’re quite real, and they require your attention, but there’s a whole range of other lubricant issues, as well.

With that in mind, we thought it would be a good time to present an oil FAQ–Frequently Asked Questions–to deal with common queries and to make sure everybody understands the basics of engine lubrication.

We can’t hope to anticipate every question you may have, but with help from various sources–including oil suppliers, filter manufacturers, engine makers, and the Technology and Maintenance Council of the American Trucking Associations–we’ve tried to highlight the topical ones as we head into a new engine era.

Q. How important are additives?
A. They’re part of an oil’s basic chemistry, added to the base stock in ways unique to the supplier in question. Essentially, they’re what makes one oil different from another. They play several roles–like anti-foam agents, anti-wear agents, corrosion inhibitors, and soot dispersants. They wear out, which is the main reason you change your oil.

Q. What about aftermarket additives?
A. A controversial question. Some experts say you can’t dump in a bunch of chemicals and hope to revive your tired oil because you can’t match the original chemistry. Others claim to have had great success with various additives and enhancers. Consult with your engine and/or lube supplier.

Q. What’s so bad about soot?
A. Soot is partially burned fuel, or carbon, but it’s a killer because it can build up in your engine oil, coat your cylinder liners, and increase oil viscosity. The result? A gummy sludge or microscopic particles of carbon that get together and form abrasive nodules creating poor engine power, cold-start problems, and maybe greater bearing and valve train wear. Soot is small, with particles less than 1 micron thick, so it passes through most filters.

Q. Can soot be controlled?
A. Yes, with a soot-dispersant additive present in all engine lubes. Soot will be held harmlessly in suspension by a good oil until its additive package wears out. By the time you drain your oil, it might hold as much as a gallon of carbon in suspension.
Soot is worse with modern engines. Emissions regulations demand retarded injection timing which loads up the oil with soot instead of burning it off and sending it out the exhaust. Hardware changes–like higher fuel-injection pressure–have helped burn the fuel more completely, but emissions demands have countered those efforts. Soot loading is now greater than ever.

Q. What’s the challenge with 2002 engines?
A. The basic issue is exhaust gas recirculation, which all heavy-duty engine manufacturers except Caterpillar are using to cut emissions from their diesels. An EGR system takes exhaust gas out of the normal exhaust flow, upstream of the turbocharger, runs it through a separate cooler, and then ships it back over to the intake manifold, downstream of the aftercooler. While EGR engines will run at higher temperatures, peak flame temperatures will actually be cooler, which means inefficient combustion, which in turn means more soot than ever. Most experts agree soot will double. Oil with the new CI-4 rating can minimize soot-related engine wear, soot-related thickening of engine oil, and the neutralization of acids created in the engine by EGR.

Q. What causes bearing wear?
A. Contaminants, acids, and cold starts. Indeed, any type of dirt that gets in your oil can scratch bearings and leave them open to acid attack. Acids come out of combustion, more so with EGR engines, so oils have detergents and dispersants to neutralize them.

Bearing wear from cold starts is a big factor. The fewer the starts, the longer engine bearings will last, but that isn’t an invitation to idle. The solution is to choose the viscosity that suits the climate you’re in. A thicker, high-viscosity oil can increase the time it takes for oil to reach the bearings on start-up, especially in cold weather when oil tends to thicken anyway, and that causes a lot of bearing wear in the first few seconds.
Your best bet: use a 15W-40 multigrade until about -15 C. Below that, consider a 10W-30 or 10W-40 multigrade.

Q. Can oil affect fuel economy?
A. Sure. All other things being equal, it’s a matter of viscosity. Oils with lighter viscosity give you better mileage than heavier oils.
For example, an SAE 30 grade is a “thinner” oil that will not resist flow as much, so it’s more fuel-efficient than SAE 40. Likewise, a 15W-40 multigrade delivers better economy than SAE 30. One oil refiner says there’s about a one-per-cent improvement in fuel economy between each viscosity grade.

However, the thinner oil won’t provide the same protection against metal-to-metal wear, so many people take the middle road and go for something like a 15W40 multigrade. Trust your engine maker on this one.

Q. Can I burn used oil in my fuel?
A. It can damage your engine and chop power by increasing injector wear, plugging filters, and forming carbon deposits on piston tops. Worse yet, it probably violates environmental regulations. If the fuel in your tanks is inspected, the oil mixture will be black and that will likely mean a fine.

Q. What should the initial drain interval be on new iron?
A. If you have a brand new truck–or one with a new engine–many veteran truckers suggest you do your first oil change at half the recommended drain interval. The idea behind that is get rid of metal particles, normal in the break-in process, as soon as you can. But there isn’t a special “break-in” oil like there was years ago.

There are countless other questions we could have addressed, but there’s only so much space. You’ll find the oil companies extremely willing to answer your oil questions, and the engine manufacturers, too. So do ask.