How to choose the right oil for your engine

Timing the Flow

Whether it’s the latest and greatest synthetic lube or a discount petroleum product engineered to meet a price point, all motor oil is tagged by its viscosity. Oil viscosity, also known as its weight, refers to how thick or thin it is; the lower the weight, the thinner the oil. 

An engine needs the correct oil viscosity to run well: The stuff must be thin enough to flow through the engine during cold starts, yet thick enough to keep lubricating when the engine is hot. Since a straight-weight oil gets thinner when heated and thicker when cooled, these two basic engine needs can be at odds with the basic nature of the lubricant. 

This is precisely why multi-viscosity oils were developed. These products have different viscosities at different temperatures; as an example, a 10W30 oil has the viscosity of a 10 weight at 0 degrees Fahrenheit and that of a 30 weight at 212 F. 

The viscosity is measured in a laboratory. An oil sample is brought to the specified temperatures and then allowed to flow through a standard-sized hole; the viscosity rating is determined by the length of time it takes the oil to flow out of the hole. If it flows quickly, it gets a low rating; if it flows slowly, it gets a high rating. 

Follow the Rules

In most cases, we recommend using the oil viscosity that was specified by the car or engine’s manufacturer. For example, if a Mazda MX-5 requires 5W20 from the factory, run that weight. If there are worries about that oil not having enough film strength for that particular car and its use, pick a better oil—don’t change from the recommended viscosity. 

Most companies have moved to lower-viscosity oils in recent years. That’s not because of tight internal engine clearances, but because of the federal government’s penalties on manufacturers who fail to achieve their average CAFE mileage standards. Moving from a 5W30 down to a 5W20 can help a fleet squeak out just a hair better mileage; it may not add up to a real difference to individual consumers, but that little improvement can save big bucks for the manufacturers in avoided fines. (Many OEs have also tightened up their engine clearances in addition to using lighter oils, but it’s been only a very slight change.) 

The best-selling oil weight in North America is 5W30—as it should be, because it’s a great viscosity for flowing through the many pathways in a modern engine. That’s why this weight is recommended by so many automakers.

Many old-school engine tuners, however, insist on running thicker oils. “We sell far too much 20W50 these days,” says Red Line Oil’s Cameron Evans. “It’s slowing down as people get clued in, but generally it is far too heavy for almost every passenger car or even many race cars.” 

Many late-model cars come from the factory with an oil cooler or are fitted with an aftermarket cooler. Track day sessions tend to be on the short side, further negating the need for the really heavy oils. A heavy oil can also leave an engine vulnerable when cold, Cameron continues. High viscosities can also trap a lot of heat in the oil, which can make a bad situation even worse.

Blazing Your Own Path

Despite the usual recommendation to follow the owner’s manual, that helpful book isn’t going to be very relevant when the factory setup has been thrown to the wind. Adding boost, significantly raising the redline, and running long track sessions can put more strain on the oil. 

In these cases, your oil temperature gauge will quickly become your best friend: Oil temperature can be a great guide for choosing the right oil viscosity. In fact, you’re going to have trouble determining which viscosity to run without a gauge.

Unfortunately, there’s a tendency to immediately jump to a thicker oil without this data. This often happens with mildly modified cars. An owner will add some basic bolt-ons—intake, exhaust, ECU tuning and so on—and then go with thicker oil because he assumes the engine is at the limit of what the factory 5W30 oil can handle. 

Thanks to that heavier oil, the engine’s oil temperatures may never rise above 200 degrees while the car is on track. As a result, the oil never becomes warm enough to do its job and slows down the engine. Cold oil also accumulates contaminants more quickly than warm oil, meaning shorter service intervals.

When experimenting with different oil weights in a car’s engine, a quality oil temperature gauge is a must-have tuning tool. The magic number for oil is 212 F—if the oil is too thick, it can actually keep the temperature too low, meaning the oil won’t provide enough protection. Photography Credit: J.G. Pasterjak

While the 212-degree mark is a great target, it’s not always realistic. So if that same owner refills his engine with the factory-recommended 5W30 oil and then sees more than 212 degrees—something like 230 or even 240 degrees—there’s still no reason to panic. That’s where a great synthetic oil can help, as it still has plenty of film strength at those temperatures. 

If the oil temperatures go beyond 260 degrees on a regular basis, that’s a clue to start looking at a different viscosity. The best synthetics are generally effective up to 300 degrees—which is where engine seals can start to fail. Heavier oils tend to carry more temperature as well. For example, if an engine is running hot with a 20W50 oil, switching to a stable 10W40 may very well lower the temperatures to a better zone.

And it’s okay to mix oil weights to sneak up on the right viscosity for a particular application. For example, mixing 5W20 and 5W30 might yield ideal oil temperatures; in other cases, running a sump full of 5W30 and 10W30 may be the right answer. 

Short of constantly pulling apart an engine to check the wear, that all-important oil temperature gauge is going to help you zero in on the right viscosity. While an old TV ad once proclaimed that motor oil is motor oil, having the right lubricant in your sump can greatly improve both performance and engine life.