CoralDoc

I understand that 5W/40 has a lower viscosity when cold and higher viscosity when hot compared to a 10W/30 oil.

However, how does viscosity relate to engine protection and performance?
I also understand that an oil with lower viscosity when cold will more easily run through oil passages, and an oil with higher hot viscosity will allow greater oil pressure compared to an oil with a lower "hot" rating. But how does this relate to engine protection? I assume that a thicker film of oil protects better than a thin one, and thicker oil is less likely to be squeezed out of small spaces like those in engine main bearings. Is that all there is to it?

Furthermore, oil with a wider spread of viscosities has more additives (viscosity modifiers) than an oil with a narrower range. Are modern oils no longer subject to this concern?

Finally, in addition to providing greater fuel economy, oil with lower viscosity has been shown to allow engines to make more power than oils with higher viscosities. This makes sense since thicker oil with have greater drag on moving parts than thinner oil.

So, if you compare an engine that uses 10W30 with one that uses 5W/40, will there be any appreciable difference in terms of engine longevity? If so, I think that it will be small compared to other issues that reduce engine life (like running an engine near redline for extended periods).

Thanks for any additional feedback.

cdelena

Honda did specify 5W40 as an alternative in the PDI documents I have seen.

Lower viscosity certainly pumps easier when cold, and higher viscosity provides a heavier boundary, but the needs of an engine vary depending upon temperature, assembly specs, wear, and usage. Anything you use WILL be a compromise.

Very broad viscosity range uses enough viscosity improvers that could leave more deposits when subjected to extended heating. I do not know if that is still true as many of these formulations have improved over the years.

I have always used 5W30 or occasionally an oil in the 0W40 range. Reasonable changes of any quality oil in approximately the right viscosity range will provide fine lubrication for 99% of uses. All of this gnashing over nits in specification really means little.

LUVNMBRS

Oils with higher viscosity have greater film strength. Thus, they are better able to withstand the high shear conditions in areas such as the main bearings and cam lobes. As temps. increase causing the oil to thin out even more, the extra viscosity provides an extra cushion of protection before metal to metal contact (ie. damage/wear) occurs. Remember, certain parts of your engine get much hotter (at high revs. & high loads) than the water temps. shown on your guages.

Additionally, superchargers & turbos add to this heat problem due to greater combustion temps.

The downside to all this is greater drag & loss of power when you don't need this layer of extra protection (ie. during normal street driving).

Nowadays, most good synoils need very little viscosity modifiers to achieve a wide range...not so with minoils, though.

s2kBryan916

I can get it real cheap if you want some. Mobile one full synthetic 5W40!

xviper

Thanks, but shipping cross border for something like this doesn't make it worth while. I'll just continue to use 10W30 most of the year and during those months where it can get down to -30*C, I'll just go to 5W30.