Through thick and thin

Max Kvidera | November 02, 2010

Specialists weigh in on cold starts, wear protection and other oil viscosity matters.

Nine out of 10 heavy-duty long-haul trucks use 15W40 oil. Despite that long-standing domination, that viscosity might not be the best choice for all conditions and applications.

Lighter oils help extend longevity of engine parts.

While 15W40 remains a solid choice for running in most temperatures and provides superior protection against engine wear, lighter viscosities offer other benefits. A 10W40, 10W30 or 5W30 oil – all widely used in Europe, where truck fuel economy standards are widespread – helps reduce fuel consumption 1 percent or more, recent studies have shown. Lighter oils might also offer better cold starting at below-zero temperatures and help extend longevity of engine parts.

Choosing the right viscosity requires knowing what your engine manufacturer recommends, ambient temperatures where you’ll be running and the needs of your application.

We asked these specialists to address questions on viscosity:

Castrol Heavy Duty Lubricants: Corey Taylor, senior heavy duty technologist

Chevron Global Lubricants; Jim Gambill, direct marketing specialist

Citgo Lubricants; Mark Betner, heavy duty product manager

ExxonMobil Lubricants and Specialties; Maria Burcham, CVL products technical advise; and

Shell Lubricants: Dan Arcy, OEM technical manager.

How important is choosing the proper viscosity?

Lighter viscosity oils, such as 5W-30, provide improved cold starting in sub-zero temperature environments while matching heavier oil performance under operating conditions.

CHEVRON: It’s one of the most important things, but not the only one. You want to make sure it meets engine manufacturer specifications (check your owner’s manual) and that it’s the latest American Petroleum Institute classification, CJ-4.

CASTROL: While owner-operators often think about what viscosity to choose, engines primarily continue to be designed to operate using 15W40 in virtually all climates.

What viscosity works best in extreme temperatures?

CASTROL: Except in very cold climates, 15W40s do well from the Mojave Desert to northern Minnesota. We do have some customers who buy our 5W-30 because of the cold start performance.

SHELL: When it’s getting down to zero degrees in Minnesota in the winter, the 15W40 is not going to be acceptable. It may mean going to a 10W30, 10W40 or 5W40. If running extreme temperatures year-round, you could select a synthetic 5W40. With the 5-weight, you could get that real low temperature performance in cold weather, and with the 40-weight, you have high-temperature film thickness you need when you’re in 100-plus-degree temperatures.

CHEVRON: If you’re an over-the-road trucker, you can feel very comfortable that a mineral 15W40 oil is the right viscosity – if you’ve got other things such as an engine block heater. Even without a block heater, our recommendation is to stay with 15W40. The oils are very well formulated and flow at very low temperatures. If running short-haul, say in the Dakotas, you might want to switch to 10W30 year-round.

CITGO: You have two choices: You can plug the engine into a block heater or use a 5W40 oil.

What are the pluses and minuses of lighter viscosities?

SHELL: You get better fuel efficiency over what you get with 15W40. In a medium-duty test we ran, we demonstrated you could get 1.6 percent less fuel consumed when using a 10W30 than with 15W40. It doesn’t sound like a bunch, but with fuel at $3 a gallon and driving 120,000 miles a year, that could mean $750 to $1,000 savings. In the long term, I think we’ll be seeing more lighter viscosity grades. It’s proven that durability is equivalent to 15W40 and you can get improvement in fuel economy.

CASTROL: A 10W30 is thinner at both ends of the temperature spectrum. Thinner oils offer less resistance to the moving parts in the engine, which turns more easily and operates more efficiently. Lower viscosity oils need to be more robust to compensate for the thinner oil. The additive packages are highly specialized to protect the engine because the oil film is thinner. In older engines (pre-2007 and especially much older models), we have heard of issues with weak oil pressure. Thinner oil is prone to result in lower oil pressure.

CHEVRON: In testing our 10W30, we found that an improved fuel economy of 1.3 percent in over-the-road trucks (versus 15W40) didn’t come at the expense of engine durability. A premium product should be able to deliver wear levels and oil drains that operators have come to expect from the leading 15W40 oils. That’s good news for operators considering a lower viscosity oil. strives to maintain an open forum for reader opinions. Click here to read our comment policy.