A reader asks: My car is supposed to use 5W-20 motor oil. I accidentally bought 0W-20 and did an oil change without noticing. Is it safe to continue running the 0W-20?
Our answer: In most cases, 0W-20 is a perfectly acceptable substitute for 5W-20. Both products are 20-weight motor oils at operating temperatures. However, the exception to using 0W-20 in your case may be if the vehicle is under warranty (see more below).
Actual Viscosity Comparison Of 5W-20 vs. 0W-20 Motor Oils
Let’s compare AMSOIL Signature Series Synthetic 0W-20 and 5W-20 by the numbers. The first specification is the viscosity measured at 100° C (212°F). This value is measured in “centistokes”. Both motor oils have a viscosity of 8.8 centistokes. What this tells us is that that at operating temperature, both products are all but identical.
- 5W-20 8.8 cST
- 0W-20 8.8 cST
Now let’s look at the viscosity number of each product measured at 40°C (104°F). Not surprisingly, the 0W-20 has a slightly thinner viscosity at this temperature.
- 5W-20 50.6 cST
- 0W-20 47.1 cST
Below are the “cold-pour-point” temperatures for both the 0W-20 and 5W-20. This test determines the temperature at which the motor oil ceases to flow. Of course, the 0W-20 has a marginally lower pour-point than the 5W-20 in this test. But in real-world driving conditions, either product offers superb cold-flow protection. Both motor oils have cold-flow thresholds in the -60°F range.
- 5W-20 -50°C (-58°F)
- 0W-20 -53°C (-63°F)
When a newer vehicle is under warranty, a dealer may have cause for denying coverage when using a slightly different motor oil viscosity. The odds of this being a problem may very well be remote. But nevertheless, it is important to keep this in mind. Some may consider switching from the recommended 5W-20 to 0W-20 for improved cold-start protection. However, the cold-pour-point numbers shown above don’t point to a tangible advantage.
Using 0W-20 in place of 5W-20 is not completely uncommon. As a rule, we strongly urge customers to stick to the engine oil viscosity suggestion in their owner’s manual. However, substituting 0W-20 in place of 5W-20 in a pinch should not compromise engine protection.