Basic question on oil grades and oil consumption

[kilou]

Hi all,

hopefully thanks to BITOG I have some understanding of the meaning of oil grades. Basically a 10W40 oil has the viscosity of a SAE40 at 100°C with some kind of cold pumpability performances corresponding to 10W. A 5W40 will also be a SAE40 at 100°C but will have better pumpability when cod compared to a 10W40.

Now I often hear people advise to get the lowest possible W-index to get better pumpability when cold to lower engine wear on startup. This of course makes sense but raises a question: the cold index (before the W) also has an impact on the viscosity at 40°C for instance! A 5W40 will have about the same viscosity as a 10W40 at 100°C but it will have a lower viscosity at 40°C.

After looking at my oil temperature gauge, I can see the oil is mostly between 60-70°C most of the time and it takes a quite long time to get to 60°C from a cold start.On the other hand, the coolant temperature reaches and stabilize at 88°C in a few minutes only. Therefore my car runs most of the time with an oil temp that is halfway between 40 and 100°C. This means that a 10W40 will always remain thicker as a 5W40, even after driving hours and the engine temperature is hot!! So it should make sense that given the same quality of basestocks, a 10w40 will always reduce oil consumption (even in a hot engine) compared to a 5w40.

Most of the time, I've heard people say the hot index (after the W) is the one to increase to reduce oil consumption (ie run a 5w50 instead of a 5w40 for example). Now does it make sense to run a 10w40 instead of a 5w40 to lower oil consumption??? Basically if I burn some oil in 10W40, my engine should burn even more on a 5w40. Does this theory make sense?

Thanks

 

[kraxxus]

Bump because I'm curious too.

I'm new to it all, but it seems if you're worried about running temp/burning oil to use the higher cold index, but since 100% engines don't burn oil, and a lot of wear can happen at startup, the lower cold index would be better. So maybe in your situation use the higher to help burn less? Not 100% sure.

 

[ARB1977]

Its not rocket science...run what ever floats your boat and see what happens. No matter what 30W i run neither car or truck doenst burn.

 

[oilyriser]

What would matter is viscosity at the particular point where the oil is actually leaking. If the drain plug is leaking, then sump temperature matters; if the rings are bad, then viscosity at ring temperature matters.

 

[bepperb]

There are plenty of 10w30's that are thinner than their 5w30 counterparts even at 40 degrees. Mobil 1 springs right to mind. My point is that anywhere from 40 to 100, a 5w and 10w will be similar in visc. It isn't until you get colder than room temp that there will probably be a difference.

For your consumption issues, you might want to try a High Mileage oil, which will probably only be available in one of those weights. I really doubt you'll see significant differences if you switch between two versions of an oil available in both weights. But I would go a full grade (30 to 40) if you live in a climate where cold performance won't be an issue.

 

[Jakegday]

what i dont understand is how the oil thickens as it gets hot. its only a 5 weight when its 5 degrees out, but it thickens up to a 30 weight as the engine runs and gets hot. i thought oil got thinner as it got hotter in the engine, can someone explain that also?

 

[72customdeluxe]

It doesn't thicken. It uses viscosity improvers in a thin oil, so basically a 0w30 is a thin oil that only thins out to what a straight 30 weight would be at operating temperature. So when cold it flows like the first weight number is, and then acts like the second number at 100 degrees celcius

 

[ffhdriver]

If you're using a 5w30 oil and the ambient temperature never gets below 40F then you engine never sees the 5W on start up right?

 

[FZ1]

You need to keep the first number low cause you are in a cold climate and want oil to flow and the engine to "warm up",I use the term loosely, as quickly as possible.

 

[Big Jim]

Originally Posted By: Jakegday
what i dont understand is how the oil thickens as it gets hot. its only a 5 weight when its 5 degrees out, but it thickens up to a 30 weight as the engine runs and gets hot. i thought oil got thinner as it got hotter in the engine, can someone explain that also?


It doesn't thicken as it gets hotter. There are two viscosity ratings involved. The cold viscosity and hot viscosity. Each are tested at the specified temperature and the cold measurements have nothing to do with the hot measurements.

It is probably better said that it thins less as it get hotter.

 

[ekpolk]

Originally Posted By: ffhdriver
If you're using a 5w30 oil and the ambient temperature never gets below 40F then you engine never sees the 5W on start up right?


Not really. The SAE rating system is horribly confusing. The "hot" rating is actually based upon the oil falling within a certain range of viscosity when it's at 100C. On the other hand, the "w" rating is NOT a direct reflection of viscosity. Instead, the oil has to pass certain tests that ensure its pumpability at low temps. This gets tricky because a low quality, but "thin" oil may actually start to solidify (and thus be non-pumpable) before a higher quality, but thicker oil does the same. In this way, you may actually have an oil with a lower "w" rating that's "thinner" by the vis numbers alone. So, especially at the cold end, you have to be careful in looking at those numbers.

 

[kilou]

Originally Posted By: oilyriser
What would matter is viscosity at the particular point where the oil is actually leaking. If the drain plug is leaking, then sump temperature matters; if the rings are bad, then viscosity at ring temperature matters.


Now that's a good point! I presume oil burning is mostly located at rings and/or valve seals. In these 2 circumstances, does the viscosity at 100°C make more sense although the oil temperature gauge reads 60-70°C? I guess the oil temp gauge reads the temp in the sump.....

 

[jorton]

"Basically if I burn some oil in 10W40, my engine should burn even more on a 5w40. Does this theory make sense?"

I think consumption will be about the same. Try using the heaviest grade that's allowed in your climate(Oil that's certified for gasoline and diesel engines is a little thicker than passenger car oil.) If you're doing that now, experiment with another brand of oil.

 

[Nickdfresh]

Not necessarily. A 5W-40 is going to be a synthetic, and in some instances may resist burning more so than a 10W-40 conventional oil would...

 

[mechtech2]

Oil gets thinner as it gets hotter.
A 10-40 BEHAVES like a 10 at 0 deg F. But that is about 20 times thicker than the oil when it is at 212 F. Much thicker.

Anyway, thick oil generally slows consumption and even some leaking. So anything thicker should use less. We are really splitting hairs about 5-40 to 10-40. Also, the 5-40 is most likely a full synthetic, and may have less net usage problems over the conventional 10w40 in the long run.

 

[Mitch Alsup]

Originally Posted By: Jakegday
what i dont understand is how the oil thickens as it gets hot. its only a 5 weight when its 5 degrees out, but it thickens up to a 30 weight as the engine runs and gets hot.


That 5W oil at 40dF is 60-70cSt
While
That -30 oil at 212dF is 10cSt

10cSt is less (thickness) than 60cSt. So, it does get thinner, it just does not get thinner as fast as a straight weight oil;

That 5W oil would be 3cSt at 212dF
That -30 oil would be 100cSt at 40dF