With today’s drivetrains needing to carry more torque, and aerodynamic tractor designs that stall airflow around components, the days of cheap mineral lubricants are over. And single-barrel approaches to transmission and axle lubrication are harder to come by.
“Because of the very different operating temperatures of today’s transmissions and axles, distinct additive systems now need to be used in each component,” says Rick Muth, manager of lubricants for Roadranger. EP – “extreme pressure” – additives are used in rear axle lubes because of the hypoid gears and the sliding friction they produce. “That same lubricant will not work in a transmission because of the higher operating temperatures,” Muth says. “The heat will activate the additive chemistry and create potentially harmful carbon deposits.”
Currently, there are no multi-viscosity engine oils that can be used in Eaton constant-mesh transmissions. “Very specialized fluids have been developed to meet the unique requirements in engines and transmissions,” Muth says.
On the other hand, engine oils that meet ArvinMeritor’s lube specs also are acceptable for the manufacturer’s transmissions, says Rick Martin, manager of training and technical publications for ArvinMeritor Commercial Vehicle Systems. But Martin agrees with Muth regarding the use of rear axle lubes in transmissions. “Rear axle lube has a GL5 additive that begins to break down at 250 degrees Fahrenheit, and some transmissions will see that temperature or greater,” Martin says. “GL5 will destroy the output shaft seal lip, damage the synchronizer and coat the inside of the transmission with sludge.”
Mineral fluids that meet Mack GO-J can be used in the OEM’s transmissions, transfer cases and axles, says Jim Nelson, Mack technician. GO-J-Plus fluids have been approved for extended drains up to 250,000 miles/2 years in highway service, and Mack does not specify that they need to be synthetic – only that they pass the applicable performance tests. These fluids come in both 75W-90 and 80W-140 viscosities, but the heavier grade is not used in the transmission.
The Mack TOA-Plus standard applies to a synthetic 50-weight fluid used in the transmission only and allows change intervals to 500,000 miles/3 years in on-highway service.
Volvo’s I-Shift automated transmission allows a 250,000-mile change interval when running a 75W-80 synthetic fluid on the 97307 approved list of fluids that meet the STB 1273.07 standard, says Greg Shank, technical manager of gear and transmission fluids for Volvo.
Transmission and axle lubes are OEM-specific because each manufacturer wants to make sure the lube will protect their equipment, says Brianne Wissel, product technical adviser with ExxonMobil. Sometimes it is possible to service both a transmission and axle with the same lube, but check with the OEM first to make sure, Wissel says. “EP additives are incompatible with some transmissions,” she says. “They can be aggressive toward certain metals.”
It’s smart to use full synthetics while the components are under warranty and if the truck sees temperatures below -15 F, says Jack Zakarian, Chevron’s global manager for driveline technology. Once warranty is up, Chevron has a suitable mineral lube “that will protect both the transmission and axle” in normal climates. “Read the current specs, and follow them,” Zakarian says. “They are changing year to year.”
Richard Gapinski, driveline technology manager for Castrol Brands, BP Lubricants, says his company feels synthetics always offer better service. “They deliver increased change intervals because of increased oxidative stability, and give increased fuel economy,” Gapinski says.
With synthetics, there is a big difference in oxidative stability and viscosity, with less shear down, so they maintain the viscosity needed to protect a transmission, says Walt Silveira, U.S. technical service manager for Shell Lubricants. “With five to 10 times the drain interval, you’ll end up recouping product cost,” Silveira says.
Mike Bradshaw, vice president of technical services for LongRider Lubricants, says he is a big proponent of synthetics because of reduced friction. “A good rule to follow – use the lowest viscosity allowed by the OEM,” Bradshaw says. “Many fleet managers equate the viscosity of the oil with protection, but this is not the case.”