I've noticed that the recommended motor oil weights for cars in the last 5-10 years are often lower than earlier models. It seems like 10w-30 used to be almost ubiquitous in the 80s and early 90s, but now I'm seeing manufacturers recommend 5w-20 and 0w-10.

What does this change imply? Could it indicate improved engine design or manufacturing techniques? Tighter tolerances?

Or is it related to something else?

  • @barbecue - that's actually correct, at least indirectly -- the lighter oil weights are used to improve fuel economy and fuel economy ratings are driven (at least in part) due to the desire to reduce greenhouse gases that contribute to global warming.
    – Johnny
    Commented Oct 21, 2016 at 18:44
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    Yeah, I was more thinking along the lines of warmer winters allowing lower viscosities.
    – barbecue
    Commented Oct 21, 2016 at 18:54
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    @barbecue - it's the other way around - warmer weather would allow thicker viscosities. For example, my car allows 15W-40 down to +5F, but for colder temps, they recommend 5W-30. So global warming would not cause a trend to thinner oils.
    – Johnny
    Commented Oct 21, 2016 at 20:22
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    @Johnny Doh, I'm an idiot, of course you're right.
    – barbecue
    Commented Oct 21, 2016 at 22:11
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    @barbecue - don't you hate it when you make a joke and some jerk comes in and explains why your joke is not technically correct? :)
    – Johnny
    Commented Oct 21, 2016 at 22:27

3 Answers 3


Two words, fuel economy.

Using a thinner oil allows manufacturers to eek out a little more fuel economy. Thinner oil flows a little easier and has a little less resistance.

To use the thinner oil, as you suggested, the tolerances inside the engine have gotten tighter. The tighter tolerances require new manufacturing techniques.


Better engine manufacturing quality control permitted less bearing clearance. Lower viscosity oils are needed in the smaller bearing clearances to permit enough oil flow to keep bearings cool ( bushing type bronze primarily but also good for rolling element hard steel bearings). Secondary advantage; lower shear resistance in thin oils makes less resistance and higher fuel economy. Also less cranking resistance on a cold start. I remember long ago , trying to start a car in exceptionally cold Chicago weather , kept the battery warm in the apartment overnight. Next morning put it in the car ; the engine turned over about twice and stopped , after market oil pressure gauge said 30 psi oil ; 10 W30 oil.


about right but the main reason for thinner oils is that of making engines more efficient and better environment friendly. To make a 4 stroke engine "cleaner" requires the engine to run or turn over faster..to do that they have to have shorter strokes which leads to a small engine and thereby smaller oil galleries to transfer oil around the rotating parts (90% of wear is produced in the first few revolutions of the crank when starting before hydrodynamic lubrication is achieved...hence the use of low viscosity oils...the 10 in 10w/30 is the oils viscosity at 40 Deg C and the 30 is the oil viscosity at 100 Deg C....So having an oil Ow/30 means the oil will get to those rotating parts a bit faster at the lower temperature

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