How Oil is Impacted by Start-Stop Engines

Almost all technologies influencing the auto industry have one common goal: the best possible fuel economy. Start-stop engines are another means car manufacturers have at their disposal to make sure vehicles adhere to tomorrow’s increasing fuel-economy regulations.

About Start-Stop Engines

The idea of start-stop engines is not complicated: when idle, the engine shuts off automatically, and when you remove your foot from the brake, the engine restarts.

This saves fuel when in an idle position. Automakers rolled out a number of start-stop systems in the late 1970s and early 80s, but drivers did not see the benefit—they found start-stop systems to be clunky and they were not willing to pay more for them.

Start-stop systems these days are more streamlined and available on many models from the majority of automakers, but this doesn’t mean they don’t still have their downfalls. Some automakers have even implemented “off” switches that disable start-stop feature due to undesirable feedback from drivers.

Start-Stop Engines Are Here to Stay

Regardless of their cons, start-stop engines are most likely here to stay. After reviewing these statistics, you may begin to see why:

  • Bearing manufacturer MAHLE says that U.S. vehicles in 2017, while idling, consumed upwards of 3.9 billion gallons of gas
  • According to Buick, using the EPA test cycle, start-stop engines can result in a 4-5% increase in fuel economy
  • Automakers constantly strive for even the smallest increase in fuel economy, so it stands to reason that they’d want to continue with technology that offers up to a 5% improvement

…So How Is Motor Oil Affected?

Perhaps you know that the majority of engine wear is an effect of cold starts, but damage can also occur during warm starts—like when your start-stop engine resumes. To learn why, we must understand the technical aspects:

The crankshaft completes thousands of revolutions per minute in an engine that is running. While spinning, oil moves through small holes in the crankshaft journals and creates a buffer between the journals and bearings to prevent wear. 

Ideally, the crankshaft will very rarely, if at all, come into contact with the bearings. This is referred to as hydrodynamic lubrication. In this situation, bearings will have an extended life and there will be little wear.

When you stop the engine, the oil loses viscosity and the crankshaft no longer has as much of a buffer between it and the bearings. The oil is easily able to settle into the minute ridges of the crankshaft. This is referred to as boundary lubrication.

Restarting the engine restores the oil film and hydrodynamic lubrication returns. It becomes important to have the appropriate oil additives in this process.

Wear Adds Up

The wear each time you start the engine may only be slight, and it’s not a major concern if you keep up with engine maintenance and use a quality oil.

So what happens if your engine’s start-stop cycles increase? MAHLE poses another statistic:

Engines with start-stop capability may produce three times the start-stop cycle than traditional engines, which means three times the possibility of wear, three times the boundary lubrication and three times the engine starts. Wear on bearings can also increase, as well.

Pieces of metal can break off and get into the oil. The surface on the bearing becomes less smooth, leading to wear as metal surfaces catch on other surfaces. The crank journal and bear can actually meld together, effectually ruining the bearing.


It is clear, then, that if you have a start-stop engine, a premium synthetic oil will assist in protecting your engine from bearing wear. Start-stop engines are more affected by boundary lubrication than a traditional engine, so it is even more important to use the highest quality oil with exceptional film strength and additive quality.

AMSOIL Signature Series Synthetic Motor Oil is the right choice. It provides 75% more engine protection against loss of horsepower and engine wear to provide protection for advanced engines of today.