Engine lubricating oil comes in many viscosities (weights) and a variety of base oils. While you should always follow the engine manufacturer’s recommendations for specification category (such as API CJ-4 or Caterpillar ECF-3), that still leaves a wide choice of oils across a range of prices.

One of the decisions to be made is whether to use a conventional mineral oil or upgrade to a more expensive semi-synthetic blend or fully synthetic oil.

What exactly is a synthetic oil?

“A fully synthetic oil is nothing more than a mineral oil that you’ve taken apart and put back together in a controlled way,” explains Jami Melani, the field engineering/heavy-duty technical services manager for BP/Castrol (BP Lubricants USA). Melani said hydrocarbon molecules as they come out of the ground are imperfect. “There are empty spaces on the carbon atoms where ideally there’s a hydrogen atom attached.”

An empty space creates a place for oxygen to attach to the molecule, and oxidation is not what you want; we’ll address that in greater detail later. “In a synthetic oil, each carbon atom has as many hydrogen atoms as there are spaces, so oxygen doesn’t attach easily to the molecule,” Melani notes.

Some oils are identified as semi-synthetic. Shawn Ewing, technical coordinator, commercial products for Phillips 66, says, “Synthetic blend – or ‘semi-synthetic’ – engine oils are a combination of mineral-base oil and synthetic oil, blended to achieve a balance of performance characteristics close to those of a full synthetic, but with a price point that remains closer to a mineral formulation.”

Whether they are mineral, semi- or full-synthetic, the base oils are then blended with additives to create products for specific types of applications, such as engine oil, compressor oil, gear oil or hydraulic oil.

Synthetic engine oils are multi-viscosity, a characteristic achieved with polymers. “Think of them like noodles – as the oil gets hotter, the polymers get longer and thicker,” Melani said. “When it’s cold, the oil is 15-weight, but as the oil heats up to operating temperature, the polymers make it flow like it’s 40-weight. So you get a thinner oil at cold temperatures to flow better and lubricate the surfaces, and when it gets hotter, the oil is thicker to offer the film strength and protection. That’s just the opposite of what occurs naturally: oil is thick when cold and thin when hot.”

Since synthetic base oils cost more, what are the benefits of using them in work trucks and earth-moving machinery? There are several reasons to choose a semi-synthetic or full-synthetic oil.

“Synthetic and synthetic blend motor oils can be of benefit in extremely cold as well as extremely hot environments,” notes Ewing. “For construction equipment operating in cold weather, synthetics will ease starting and reduce wear at start-up because synthetics have superior cold cranking and cold flow characteristics.”

Synthetic oils generally have a lower cold viscosity rating, 5W-40 or 10W-40, compared to the common 15W-40 mineral oil, says Stede Granger, OEM technical services manager for Shell Lubricants. Shell also has introduced a full-synthetic heavy-duty diesel engine oil with viscosity rating 0W-40 for extreme cold conditions in Canada and Alaska.

Extreme heat and high operating temperatures can accelerate oil oxidation.

“Oil exposed to air and heat combines with oxygen to form acids, insoluble sludge, and varnish,” Phillips 66’s Ewing explains. “The oxidation process leads to a vicious circle of increasing the viscosity of the oil which increases fluid friction and heat which accelerates the rate of oxidation.”

“Synthetic oils have better oxidation protection and better resistance to thermal breakdown at higher temperatures,” Shell’s Granger notes. “If you are expecting higher than normal operating temperatures, a synthetic oil is desirable.”

Another benefit of using synthetic base oils is the possibility of extending the drain interval.

As Ewing explains, “Since oxidation byproducts and contaminants are the most common reasons for an oil to reach its condemning limit upon oil analysis, the superior oxidation resistance of synthetic and synthetic blend engine oils is a key factor in their ability to extend service life.”

Lowering maintenance costs overall is the main benefit of extended drain intervals, according to BP/Castrol’s Melani. “Although the synthetics cost more per gallon, you gain from running the oil longer between drains.” Side benefits include more uptime, less oil to handle, less clerical ordering, less downstream waste, fewer filters and fewer technician labor hours.

Opinions vary about how much longer drain intervals can be extended by using synthetic oils.

“Synthetics can offer longer lubricant life in most situations,” says Ewing. The number of hours or miles increased depends on the type of service and application. However, Ewing warns, “Not all situations allow an oil drain extension just because of a switch to synthetic oil.” He cites as an example extremely dusty environments where if air filtration is compromised, dirt can enter the crankcase and cause engine wear. In this case, synthetic fluids become just as dirty as conventional motor oils.

“One of the reasons you drain engine oil is because of contamination from the combustion process,” says Shell’s Granger. “In a diesel, one of those contaminants is fuel soot. At some point, you get so much soot, no matter how good the oil is, you have to drain it.”

Nevertheless, in some circumstances, the run time between drains can be extended 50 percent or more by using a semi- or full-synthetic oil.

“We encourage customers to be responsible, step up the interval in increments, and rely on used oil analysis to verify that they are extending drains safely and responsibly without any damage to the equipment,” says Melani.

Is it okay to mix mineral and synthetic-base oils, or should the previous type be flushed out of the crankcase? The experts agree there should be no compatibility issues if the engine has been operating properly with no mechanical problems. “Synthetic and synthetic blend engine oils are blended to be compatible with conventional engine oils as well as being compatible with seals and gaskets,” says Ewing.

The quest for better fuel economy for on-highway vehicles will likely carry over to off-road equipment, says Granger. The trend is for engine manufacturers to allow lower viscosity oil to help save fuel.

“Going from 15W-40 mineral oil to a 10W-30 semi-synthetic, you pick up a little better fuel economy from thinner oil,” Melani notes. “It’s hard to measure, but intuitively, you know it takes less power to pump thinner oil through the engine.”

Full-synthetic oil is more expensive, so Melani says to consider: “What conditions does the oil have to address? Which oil will do the best job in that situation? Can I use a less expensive mineral oil and change it more often?”

Granger advises, “The choice is what really fits for the customer, what the equipment is, how old it is and in what environment it operates.”