Synthetic oils historically have offered two advantages over mineral lubes: less thickening at cold temperatures and better high-temperature stability. On the downside, they cost more. Now that sophisticated processing has greatly improved the high-temperature stability of the best mineral oils, deciding whether to pay extra for synthetic oil is tougher than ever.
An oil’s base stock determines viscosity at low temperatures (the “15” in 15W-40). Additives, including viscosity index (VI) improvers, finish the oil product. The oil must be thick enough at 200-plus degrees (the 40 part of 15W-40), protect the engine from acids produced by imperfectly burned fuel and keep soot suspended.
Synthetics have the advantage of a higher VI, says Michael Ragomo, adviser at ExxonMobil Lubricants and Specialties. “There’s less thickening of a synthetic base stock as it cools and a more gradual thinning as it heats up,” he says. That’s why it’s possible to make a 5W-40 synthetic oil that’s significantly thinner than 15W-40 when very cold, yet just as thick at more than 200 degrees. So synthetic oil has big advantages when starting a cold engine, especially in extreme temperatures.
IMPROVED MINERAL OILS
The other big advantage of a synthetic is resistance to oxidation – the tendency for prolonged operation at high temperatures to make an oil thicken. That thickening means sludge under extreme conditions, but it may also thicken the oil enough to hamper its flow to the moving parts.
However, times have changed. Mineral oils formerly made with Group I base stocks, which are solvent-refined, are now often made with more sophisticated hydroprocessing. This process separates impurities such as sulfur, nitrogen and wax, producing Group II base stocks, says Ralph Cherillo, technology adviser at Shell Oil Products.
According to Matt Ansari, heavy-duty automotive manager at ChevronTexaco Global Lubricants, “With the advent of Group II base oils made with Chevron Isosyn technology, these impurities, which used to consist of at least 10 percent of the base oil, have been reduced to less than 1 percent. Once the oxidation problem was resolved, it became almost impossible to distinguish between synthetic oils and mineral oils in terms of fighting the hostile environment in an engine.”
Cherillo agrees. “If you refine mineral oils enough, they resemble synthetics,” he says. Ansari says not only do the better mineral oils made with hydrocracked base oils not break down, they do a better job of handling corrosion and soot.
Chevron research compares the performance of Chevron Delo 400 15W-40, made from Group II base stock, with a 5W-40 synthetic oil; oil changes were made at 45,000 miles. It shows the mineral oil performs as well as or better than the synthetic oil in fuel economy, oil condition (including viscosity) and engine wear. Tests of Mobil Delvac 1 5W-40 have shown fuel economy improvements of more than 2 percent over 15W-40 oils.
Heavy-duty truck synthetic oils still hold key advantages over mineral oils, which now have enhanced high-temperature stability.
WHEN THE HEAT IS ON
Synthetics may resist oxidation better than even the best mineral oils in extreme heat applications. “We have seen some fleets in applications that run hard at 130 degrees outside temperature with sump temperatures of 275 to 285 degrees,” Cherillo reports. “In such extreme conditions, we worry about mineral oils holding up.”
Ragomo points out that the oil sump is a critical part of the oil cooling system, adding to the capacity provided by the oil cooler. And, if the pan should get sludged up because of oxidation due to high heat, “that will inhibit rejection of heat through the pan.” And, he says, “Our sludge resistance is beyond what’s possible with a conventional product.”
Therefore, very demanding high-temperature applications deserve consideration as candidates for synthetic oil. At the same time, it’s clear the most highly refined mineral oils now have a much greater resistance to extreme temperatures than in the past.
The one situation where the synthetic has a clear advantage is when you shut down overnight in temperatures below 5 degrees. Even so, there are ways to cope with frigid temperatures. Gordon Bow, who lives in Oakfield, N.Y., near Rochester, says he gets by without synthetics in the cold by not shutting down for more than five hours or by using an engine heater.
Starting, however, isn’t the only cold-temperature issue. “The pivotal difference is that mineral oils can’t have the performance of synthetics over a high range of temperatures,” Ansari says.
Cherillo says, “With synthetics, cranking rpm is higher, and there’s less resistance to flow – it pumps quicker and easier. There’s less wear at startup, and you can start at lower temperatures. Mineral-based 15W-40 oils are good down to 5 degrees. With synthetics, there’s as much as a 25 degree advantage.”
Ragomo says, “With the oil in the sump at cranking, it takes time for it to get to all the parts, especially the turbo and the valvetrain. We did tests at extreme temperatures in which it took mineral oils three minutes to reach the rocker boxes, while it took less than one minute for synthetics. The operator doesn’t know where the oil is, and he can be pulling down the highway without oil getting to the valvetrain.”
The locations in blue are where ChevronTexaco estimates there are enough days below 5 degrees to make switching to a synthetic oil worth the cost.
Ragomo says ExxonMobil has measured “the cranking benefit as 10 to 18 degrees, with an even greater benefit in terms of pumping.” This means that comparing a 5W-40 synthetic oil with a 15W-40, the engine will crank as fast even when the engine is more than 10 degrees colder.
Even though almost all of the oil is in the sump, the thin film that remains on the parts has the same viscosity as the rest of the oil. At frigid temperatures, oil with a lower viscosity carries a whopping advantage when it comes to friction. This, in turn, means the engine cranks faster. Faster cranking means higher compression temperatures and quicker starts.
“In arctic conditions, synthetics are a necessity,” Ragomo says. “But even under more favorable conditions, cold starts will produce less engine wear and make less of an impact on charging and starting systems.” A higher cranking speed will, for example, reduce starter amperage draw. So, if you live in a cold climate and historically have had high electrical repair costs, synthetic oil could help.
Oil makers agree on the cranking and protective benefits of 5W-40 synthetics. Ansari recommends basing a decision to use them not on the coldest temperature your engine will see, but on where you’ll be starting because what matters most is how frequently you see temperatures well below 5 degrees.