Fleets that do city deliveries are increasingly turning vehicles off during deliveries rather than letting them idle. While this saves fuel and cuts emissions, and in many cases conforms with local regulations, Chevron warns that it could be affecting your engine oil.

Chevron representatives say they have been seeing an increase in severe-duty operations due to these changing driving patterns, and they're not sure whether fleets and drivers are considering what this might mean to their maintenance practices.

"Customers say, 'I thought as long as I buy a premium oil I'll be in good shape,'" explains Jim Gambill, Delo brand manager. Of course, he says, Chevron agrees that buying a premium oil is an important step, "but we want to get a little more specific about those implications and what customers should look for."

So Chevron researched the issue, running an engine through stop-and-start cycles emulating this city delivery pattern at the independent Southwest Research Institute in San Antonio, Texas.

It found that there's a nearly 30% increase in engine temperature when you turn off the engine, then start it back up. "It doesn't last long, but it pops up and then drops. With that increase in temperature every time you turn the engine off and on, you can actually shorten your oil life by as much as 20%," Gambill says.

The rule of thumb is that each 10-degree increase in engine temperature can cut oil life in half.

That's because heat increases the rate of oxidation in the oil. When oil oxidizes, it forms acids. At one time, Gambill explains, those acids were all caused by the combustion process. But changes in engines and oil formulations mean that now as oil degrades, it forms acids. And you have to manage those. If you don't, taken to an extreme, those acids can cause corrosive wear in the engine.

In more practical terms, what it may mean is a change in oil drain intervals or oil formulation.

"We're seeing people are changing the way they use the truck but not changing things like the drain interval," Gambill says.

Oil analysis is important here, and Chevron recommends looking at TAN (total acid number) as well as the more traditional TBN (total base number.) TBN measures the ability of an oil to neutralize the acid, while TAN is the actual measure of that acid in the oil.

"Your TBN might be fine, but if you're building up acids, that's not a good thing," Gambill says.

So in addition to using a premium oil that's able to stand up to those acids, he says, you might have to reduce your drain intervals.

Gambill says to be aware the the OEM recommended oil drain intervals may not be right on target in this case.

"OEM recommendations often vary based on fuel usage," he says. "In severe duty, you have more fuel consumption than normal service. But in this case, you're turning the engine off and on to save fuel, you're cutting fuel consumption, but it's actually more severe. It's a little harder on your oil."

In addition, the timing on turning off the engine can make a difference because of something called heat soak. You need to idle down your turbocharger before shutting the engine town. Turbo bearings are by far the hottest in the engine, Gambill explains. So if you just turn the engine off immediately after stopping, and let the oil sit in these very hot bearings, you may "cook" the oil.

Most OEMs, he says, recommend one to three minutes of idling before shut-off. Most fleets have engines programmed to shut off after three to five minutes, but make sure drivers know about the need to idle down the turbocharger so they don't prematurely turn the switch.

So look at your specific situation, do oil analysis, and talk to the OEM and oil provider to see if you need to make any changes in your oil spec or your drain intervals.