Lube that Lasts

John Baxter | January 04, 2011

Synthetic lubricants offer long-term cost benefits and better performance in extreme temperatures.


Synthetic oils cost more than mineral oils but save money over the long run. Beyond the cost benefits, synthetics deliver superior performance under most temperatures and operating conditions, say their makers.

One advantage of using synthetic oil is the reduced downtime that comes with having extended drain periods.

While synthetics are more than twice as expensive as mineral lubes, that’s only part of the story. The recommended change interval for a mineral lube in a drivetrain component may be 125,000 miles or less, while the synthetic lube interval is 500,000 miles. To hit a million miles, you’ll replace a synthetic lube only once, while you’d replace a mineral lube seven times. You’ll also spend more on downtime and disposal.

Synthetic oil base stocks enable refiners to create engine oils with “better thermal stability,” says Maria Burcham, a technical advisor with ExxonMobil. This means oil changes are extended because heat won’t break down synthetics as fast as mineral lubes.

Thermal stability is critical in keeping oil viscosity high enough for proper component protection, especially when running under heavy loads at high temperatures. This gets complex in engines, where extending changes presents more problems because of all the by-products of combustion that inevitably end up in the oil. Sometimes the accumulation of acids and soot means the quality of the base oil will have little effect on when a change will be required.

Still, it’s clear that synthetics are ideally suited for use in the transmission and axles, where combustion by-products are not part of the equation.

Another advantage of synthetics is cold performance. Though they’re every bit as thick or “viscous” at normal operating temperatures as mineral lube, when starting out on a cold morning, synthetics will make shifting much easier because they start out thinner.

“Synthetics offer both superior low temperature performance and a high level of film strength at high temperatures to better protect all moving parts,” says Dan Arcy, a technical manager at Shell Lubricants. The fluid will be thin enough to flow easily and quickly reach all parts when you start up on a cold morning, yet it will remain more than thick enough when running at high speeds on a hot day.

Component makers stand behind their belief in synthetics’ extra protection by offering extended warranties – typically up to 750,000 miles on drivetrain components. The extra protection helps guard against component problems, protecting companies from having to pay warranty claims. You can typically keep the warranty in effect if you change the fluid, using an approved lube as the replacement, at 500,000 miles. In addition, gearboxes end up looking a lot cleaner inside at overhaul when synthetic lube has been used.

Arcy says one circumstance where the use of mineral lubes in drivetrain components might be preferred is when contamination is high, as when working in extreme dust. In such cases, inspect the lube daily and replace it as frequently as it gets dirty or diluted with a less expensive mineral lube.

Arcy also notes the critical difference between transmission and axle lubes and the importance of using the right one. Transmission gear teeth are generally flat and the gears sit directly across from one another. In a drive axle, the large ring gear sits at the level of the wheel bearings, but the pinion gear that drives it is off-center and is at 90 degrees. The gear teeth in the drive axle rub together much more, requiring special extreme pressure additives.

EP additives work fine with parts in a differential, but transmissions have synchronizer clutches made of yellow metals like brass and bronze. The EP additives can corrode those parts and cause serious damage, and the higher operating temperature of the transmission can accelerate the effect. Additives in transmission lubes, unlike those for drive axles, are adapted to the heat.

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