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I have often heard that high viscosity oil (higher than what the manufacturer recommends) is good for old engines, engines that are nearing the end of useful life.

So, for example let's consider a 1989 Opel Vectra (a car I used to have). The manufacture recommended 5W-30 for this particular car. However, when nearing the end of its useful life, it's possible to use a higher viscosity oil such as 5W-50 (I'm not sure if 5W-50 was available in 1989, as the difference between 5 and 50 is great). It should have the same cold start viscosity, yet the viscosity in operating temperature is higher.

Is there any benefit in doing this? I think that because an old engine may have worn parts having bigger gaps, a higher viscosity could be beneficial in protecting the engine. But is it really?

Let's limit the viscosity changes only to operating temperature viscosity changes. Of course you'll run into problems if using SAE 50 in a cold winter.

  • Increase the lower number also for more benefit of higher viscosity. 10w40 or a 15w40 – Moab Sep 5 '18 at 21:08
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The reasoning behind this is that as your engine's internals wear on each other, a thicker oil would find it harder to seep through into places it shouldn't be than a thinner oil. On the other hand though, those parts that have not worn out will suffer because the thicker oil may not always lubricate them properly.

Although in both instances, that was more of a concern back in the days when there weren't such things as multi-grade and synthetic oils.

The right thing to do though is keep using the grade of oil the manufacturer says to use, but check your oil levels every week or two. Because a high mileage engine may burn a bit more oil than a new one. Although you should hardly notice an increase in consumption. Let's say a new Honda engine burns a quart (~500ml) of oil between services. If it's really old, it may be burning two quarts. That is out of about a gallon (~4 liters) of oil you put in.

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    I made one small change to make this generic for the question. – Pᴀᴜʟsᴛᴇʀ2 Sep 6 '18 at 15:37
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IF the engine bearings are worn as indicated by lower than specified oil pressure : then a somewhat higher viscosity may help , like 10W 40. It will not help low compression / worn rings.

  • It may also reduce oil consumption. An oil system full of 10W-40 is probably better protected than running below the minimum level on the "correct" oil grade! – alephzero Sep 5 '18 at 19:06

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