Can thin oil sufficiently protect my engine?
Tom Vondrasek | 27th May 2023 | 5 minutes to read
Engine oil weight, also known as viscosity, has been getting thinner as time goes on, partly due to automotive manufacturers experimenting with oils of thinner viscosity to get increases in fuel economy.
Better fuel economy is obviously a positive for the average driver, especially with fuel prices getting consistently higher and tending to fluctuate wildly depending on what is happening in the world. Having unstable prices hurt everyone’s hip pocket as most of us need to use a car.
Another major benefit of better economy is burning less fuel means you are reducing the number of pollutants your car produces. With that in mind, it makes sense for car companies to target these factors when designing an engine, but what does it mean for your engine oil?
There are a number of important factors to consider when using a thin engine oil:
Oil Weights Explained – What Is Oil Viscosity?
Oil viscosity is how thick or thin an oil is and how it flows at a given temperature. There are 2 numbers on every engine oil bottle that tell its viscosity. In a 10W-40 bottle of oil, the 10W is for when it is cold and the 40 is for when it is hot, which can act as a good guideline for using the oil in winter vs summer.
For this scenario we only need to know that a 10W-30 is a thinner oil than a 20W-50. Basically, the lower the numbers listed, the thinner the oil. For a more detailed explanation on oil viscosity or weight numbers see What Do the Numbers on Engine Oil Mean?
Do Thin Oils Improve Fuel Efficiency?
Your engine is a lot of rotating, moving, hot metal parts that need to be lubricated to reduce friction and stop them welding themselves together. Engine oil provides a film over these moving parts to help cool them and stop metal contacting metal. The oil is being continually pumped through the engine, so friction doesn’t cause it to burn or get too hot.
Once oil has passed through the piece of engine it needs to lubricate it falls back into the sump for a rest. Then it is put through a filter and pumped back through the engine. This is repeated constantly.
When an engine is designed, they take the type of oil and viscosity into account. The metal moving parts have tolerances or put simply, how tightly or loosely the parts fit together. The smaller the gaps between parts the tighter the tolerances, the larger the gaps are, the looser the tolerances. Note that tolerances will vary as when an engine heats up the metal expands and that this needs to be allowed for in the design.
Using a thinner oil allows for tighter tolerances as it flows more readily, while thicker oil needs larger tolerances. That’s why modern engines use a low viscosity oil (thin when cold or hot), and older engines use a high viscosity oil (thick when cold and hot).
As oil gets hotter it gets thinner, decreasing the amount of film strength between the moving parts. The less film strength the higher the temperature. The higher the pressure and temperature, the higher the friction, which increases the wear on components.
The thicker the viscosity or weight of oil, the more resistance it creates to movement. Picture stirring a milkshake versus stirring a thickshake — they’re both similar products but the thickshake is harder to stir.
By using a thinner oil, the resistance is decreased, and engine parts will rotate and move more freely. This means less energy is needed to do it and keep it running and that improves efficiency. This in turn increases fuel economy as there are a lot of moving parts to the engine.
What Are the Risks of Using Thin Oil?
The main risk is to engine tolerances as explained above. Too thin an oil in an engine designed for a thick oil means the gaps in the moving metal parts will be too large and the oil will drain away too quickly; especially if the engine is worn, this will make it worse. The oil draining away means it won’t protect the moving metal parts as effectively and generate excess heat and premature wear on the metal surfaces.
The bad part is that the symptoms for this may not be easy to spot. The engine may appear to sound noisier than before — not exhaust noise but mechanical noise when listening in the engine bay — which will get progressively louder the longer the oil is left in the engine.
There will likely also be the smell of burning oil caused by friction as the parts don’t have enough lubrication. Other things you might notice are lower oil pressure if your vehicle is fitted with an oil pressure gauge and an increase in oil usage as some of the oil is being burnt (which may appear as a wisp of blue smoke from the exhaust).
If you already have a minor oil leak, then the thin oil might cause this to suddenly become worse.
None of these things may be easy to spot or interpret as ‘Oh, I must have used the wrong oil’. Things will get progressively worse the longer the oil is in the engine. This is all the more reason to take care when choosing an engine oil.
Can Thin Oil Be Used To Protect Your Engine?
That is an absolute yes if the engine was made to use a thin oil. I cannot stress enough the importance of following manufacturers’ recommendations when it comes to choosing thin engine oil. The viscosity numbers or oil weights are critical to the health of your engine and if you play with viscosities and something goes wrong with the engine, there goes your factory warranty.
Make sure to double check the oil’s viscosity before buying it and if you need to buy oil or get advice on the right oil for your engine, head into your local Repco store and talk to one of our friendly staff.
