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.