There are a lot of rumors about motor oil out there. But which stories are true and which are simply myths? This article will tackle four of the most common oil anecdotes and explain whether each is fact or fiction.
1. Synthetic motor oil is too slippery. It causes roller bearings to slide instead of roll, and that causes the bearings to fail.
Fact or Fiction? – Fiction
If you have ever flown on a jet airplane, you have enough experience to debunk this myth. All jet turbine engines utilize rolling-element bearings, and every jet turbine engine runs on synthetic oil. In fact, only synthetic oil can handle the high speeds and extreme temperatures found in turbine engines. This myth is very popular among the motorcycle crowd, and the roots of this myth are based in the misapplication of passenger car motor oil. The power density of motorcycle engines place greater shear forces on the motor oil than passenger car engines do. As a result, most passenger car motor oils are not appropriate for use in a motorcycle engine. This is especially true of passenger car motor oils optimized for passenger-car fuel economy. These oils are the least shear stable, and should not be used in motorcycle engines.
Failures in motorcycle engines have long been blamed on synthetic oil. However, the problem was is not the synthetic base oil, it’s the fact the synthetic oil is not formulated for a motorcycle engine. A properly-formulated synthetic motorcycle oil will provide superior performance in a motorcycle engine. Likewise, a properly-formulated synthetic passenger car motor oil will provide superior performance in a passenger car engine as well.
2. Flat tappet engines can’t use synthetic oil because the lifters won’t rotate – the synthetic oil is too slippery.
Fact or Fiction? – Fiction
This is a variation on the first myth that has become popular since flat tappet camshaft failures began to increase about 10 years ago. Like the first myth, the origin of this one also relates to misapplication of passenger car motor oil. About two decades ago it was common for racers to use off-the-shelf motor oils in their racing engines. At that time, these motor oils contained enough ZDDP (aka Zinc) to protect the aggressive camshaft designs found in racing engines. Because NASCAR teams raced for hundreds of miles each weekend at very high oil temperatures, it was common practice to use synthetic motor oils.
You could purchase premium synthetic motor oil right off the shelf back in the early 1990s that was capable protecting a flat tappet race camshaft. By the time 2005 rolled around, however, the ZDDP levels in off-the-shelf motor oils had been reduced due to EPA regulations for passenger cars, and this reduction in ZDDP found these formulations to be deficient for protecting flat tappet camshafts. As a result, the racers using off-shelf-motor oils began having camshaft failures, and because many racers used synthetic passenger car motor oils, it appeared the cause was synthetics.
Today, every NASCAR team still runs engines that use flat tappet cams, and every one of those flat tappet engines are lubricated with synthetic motor oils. However, these synthetic motor oils are special formulations with more ZDDP to protect the flat tappet camshaft. Again, misapplication is often the cause for problems that appear to be oil related.
3. Once you use synthetic motor oil, you can never change back to conventional oil.
Fact or Fiction? Fiction
An engine running conventional motor oil can change to synthetic motor oil, and you can change back to conventional if you would like. The likely start to this myth stems from an actual good practice – it is not a good idea to switch back and forth between different brands of oil. You’ve probably heard an old-school mechanic tell you to pick a brand and stick with it. That is a pretty good idea. However, it is more important to use the correct type of motor oil for your engine.
Most high performance and racing engines are actually initially broken in on conventional motor oil (specially formulated for engine break-in) and then switched to properly-formulated synthetic motor oil for use after the break-in process. Again, the key lesson here is to select an oil formulated for the specific needs of your application and then stick with that product.
4. Synthetic oils are bad for engines with old seals.
Fact or Fiction? Fact
You thought all of these would be fiction didn’t you? Well, this one turns out to be true in most applications. Notice we said most applications. While some exceptions to this rule can be found, the majority of times this rule does apply – don’t use synthetic motor oils in old engines with original seals.
When we say old engines, we mean engines that were built before 1992. Synthetic base oils are not compatible with many of the traditional seal materials, and even with “seal conditioner” additives, synthetic oils are harder on traditional seal materials than conventional oils. To avoid leaking seals, avoid very light synthetic motor oils in older engines. The low viscosity and resulting free flowing nature of the synthetic makes it easier for the oil to find a leak path. Higher-viscosity oils tend to leak less. Thus, most old-school hot rodders use thicker-viscosity conventional oils like a 15W-50 or 20W-50 in their engines.
Modern engines and modern seal materials are designed to be compatible with synthetic motor oils. This is so you can use a synthetic in a freshly rebuilt Small Block Chevy for your 1969 Camaro. If you have a ‘69 Camaro with the original seals though, then you should use a conventional oil. Now if you also happen to have a 2013 Camaro, you should use a synthetic motor oil in that engine.
Hopefully all of this puts to bed any worries or fears related to any myths you’ve heard about oil. More importantly, we hope you now see the importance of selecting the proper type of oil for the needs of a specific application. Just like a tailored suit fits the person it was tailored to better than an off-the-rack suit, application-specific formulas provide a better fit for the unique needs of performance enthusiasts than off-the-shelf motor oils.