Common Misconceptions about Viscosity and Engine Oil Grades
Many people believe that “thicker” oil is better for their vehicle. This is the furthest thing from the truth! Over the years, engine sizes have been reduced dramatically. With this size reduction, we can infer that the clearances within the engines have also decreased. Hence, a “thicker” oil from 50 years ago will not suffice in a modern-day engine.
Think of trying to drink molasses with a thick (or wide) straw. This may be possible (although challenging), but if we swapped the thick straw for a thinner, narrower straw, the person would have to use significantly more force to pull up the molasses. A similar phenomenon occurs with engine oils.
In modern engines, the oil lines are narrower, so trying to force a heavier-weighted oil (such as straight 50) would put more pressure on the engine. This is where we begin to see leaks in the engine, particularly at the bottom of the sump near the seals, where the most pressure is exerted during start-up to pump the thicker oil to the top of the engine. However, if we used the correct viscosity of the oil, the engine would not be subjected to this amount of additional pressure. So “thicker” is not always better.
Another common misconception is that the number in front of the “w” in a multigrade oil represents the thickness of the oil, and if it’s zero, then it must be very thin! The number in front of the “w” for multigrade oils represents the viscosity of the oil at start-up conditions (typically 0°F or -17.8°C for Winter).
Hence, the lower the number, the faster the oil will flow at startup. As such, a 0w20 will get from the bottom of the sump to the top of the engine faster than a 20w50. In this case, the 0w20 will provide more protection during startup compared to the 20w50, as most wear occurs during this period.
On the other hand, the number behind the “w” indicates the viscosity at operating temperature. This is where a higher number may not always be agreeable, depending on the year of manufacture of your engine or the ambient conditions. When deciding which oil to use, both numbers (in front of the ‘w’ and behind the ‘w’) are important.
Find out more in the full article featured in Precision Lubrication Magazine.
