Think thinner and synthetic for motor oils, lubes

There’s no harm in staying with the tried and true, but you’ll lose some benefits.

August-September 2019

Tom Berg


Think “low vis.” Low viscosity, that is, for motor, transmission and rear-end oil, and probably grease in the hubs. And embrace synthetics, if you haven’t already.

Low-vis synthetics and semi-synthetics are where the lube world has gone, at least for modern engines, gearboxes and wheel ends, and the reason is fuel economy, according to the experts. Federal fuel economy standards and greenhouse-gas emissions limits have caused manufacturers to design hardware that no longer requires thick lubes to protect them. Thinner lubes still take care of the components’ intricate innards, but they offer less resistance to rotating bodies, so less power and fuel is needed to spin them.

Take motor oil. For many years 15W-40 was king for truckers, and still is for many of them, said Paul Cigala, an applications engineer for commercial vehicle lubricants at ExxonMobil.

“Their dad did, their grandfather did, so 15W-40 is what they use. But 10W-30 now is preferred for newer vehicles to meet EPA regulations and fuel economy. It still protects the engine, but can improve fuel economy by 1.5% to 2%,” he said. “Fuel costs are on the downside again, but it’s still a major operating expense and it’s wise to save fuel. There’s no harm done with 15W-40, but they lose the fuel economy benefit.”

All 10W-30 oils must pass the same tests for wear protection, oxidation and thickening demanded by engine manufacturers, Cigala added.

“All development we now do is on lower-viscosity oils, and any improvements will be in them. So 30 weight is going to perform very similarly to 40 weight. For older engines or those having a fuel dilution issue, yes, thicker oil is proper,” Cigala said. “Otherwise, use 10W-30 or whatever your engine manufacturer recommends.”

Transmissions, too

That’s also true for transmissions.

“Transmissions have gotten a lot more interesting,” Cigala said. “It used to be one size (of oil) fits all. Now there are automated manuals and full automatics. And every OEM has its own requirements. Paccar has one. Eaton has one. Detroit, Volvo, Mack and all the automateds have their own. Some will take a lower viscosity oil, and require additives, depending on the makeup of the gears and bearings. In the past, high sulfur (content) protected the gears, but now sulfur can attack the bronze, aluminum and other metals in newer transmissions. There could be metal passivators (a corrosion inhibitor) required that protect the metals from acids and heat, and friction modifiers and synthetic base stocks that add life and handle the high temperatures and stresses.”

Allison automatics come in models for vocational, for delivery, and even over-the-road, that take the same fluid but oil drain intervals will differ. All (lube) products are now full synthetics, along with lower viscosity, usually 40-weight. Transmission operating temps range from 150 to 220 degrees. Many automatics, especially Allisons, have oil coolers, but with aerodynamics on tractors, air flow must be designed so it moves toward the cooler. Nonaerodynamic trucks allow air flow past the transmission and less external cooling is needed, especially with manuals.

Likewise for differentials and rear axles, which also used to require 75W-90 lube to protect the gears from shock. The heavier lubes are no longer required for axles in late-model trucks. Lower-viscosity lubes now protect them, and lower friction can save 1% to 1.5% in fuel. Because they’re synthetics, with them comes the opportunity for extended drain intervals. Differentials can go 500,000 to 750,000 miles before their lubes need to be drained and replenished with fresh product. That can be the full life of a vehicle bought new by an owner-operator, Cigala said.

Extended oil drains

Owner-operators tend to be conservative and stay with the tried and true, not just in oil types but in drain intervals, he said. Fifteen thousand miles for motor oil is an old, safe standard that many owner-operators still follow. Yet engine builders advertise 40,000 miles and more, and it’s perfectly fine, particularly when combined with regular oil analysis. An oil change done at a commercial shop costs upwards of $300, and doing it once instead of three times in 45,000 or so miles saves – well, you do the arithmetic.

Regarding oil analysis, “regular” is the key. “Some owner-operators and small fleets will send in a sample when they think they have a problem,” Cigala said, “but that doesn’t work. You have to have a history of samples to see trends – more wear metals, high oxidation, dilution, the appearance of potassium and sodium,” the latter from coolant getting into the oil, which suggests leaks and the need for repairs. Each engine establishes a history, and analysis labs send reports and issue warnings when something different and negative develops.

It is important to be educated on these maintenance issues.

“Understand what you have in the vehicle and read the owner’s manual so you know what’s recommended,” he advised. Much information is also available online, using brand names and model designations, which will send you to various websites where more info comes up, from recommended lubricants all the way to maintenance instructions. Given the complexity of today’s trucks, however, chances are you’ll have the work done. The quality of the shop and knowledge and skill of its technicians then become vital, so choose wisely. LL