I've just been reading about oil grades and now I'm totally confused! Reading through all of what I've read may clear it up, but I haven't really got enough time. http://63.240.161.99/motoroil/ seems to know alot about it.
One thing that really confuses me though is that a lot of people say that synthetic oils, such as 0W40 or 5W40, can be too thin for certain engines. Acording to this document something such as 15W40 when warm will be thinner than 0W40 when cold. The problem with cold oils is that they are too thick. So why is it that when an oil is cold and very thick it can be too thin? Is it because the engine is cold and the tolerances aren't quite what they should be?
It's not really for my Sei. It's just something I'd like to understand really, and it will probably be useful when I get my next car. I think I'm gonna stick with 10W40, or maybe go back to 15W40.
As I understand it, in the case of oil specified as 10W40, this means that it's 10 grade oil when cold (W= Winter) so it's not too thick to circulate. Then when it's warmed up the polymers come into play to make it 40grade oil, ie it's thicker when hot than when cold. Don't ask me how they do that though. Magic I expect.
Incidentally, looking at an oil application chart recently, I notice that
20W50 is rated for temps down to -10C, which we rarely see in UK, so since it's cheaper nowadays due to being out of fashion, that's the oil for the old bangers.
When you next change oil, compare the viscosity of the hot stuff running out to the cold stuff you put in. You'll notice it doesn't get thicker, so there isn't any need for any magic.
You should be using what the maker specifies and not guessing.
Oil grades come in varying numbers, so you get a 10 weight oil, 20 weight etc. The acrual viscosity in centistokes is measured at a prescribed temperature and that is a single straight oil. The oil grade is determined by bearing clearances, shaft speeds, loading, desired film stability yadda yadda. Forget why, just accept that a number is arrived at. This is the grade needed when the engine is hot.
Now in winter an engine that wants say a 30 weight oil could find that it gets so cold and viscous that it is too thick to start, and once started it flows so slowly that bearings get oil starved. Consequently it needs a thinner oil so it might have a 10 weight oil in winter.
Of course, that oil is too thin when the engine gets up to temperature, epsecially in summer when the engine will run hotter.
So some bright spark came up with index improvers. These effectivly increase the viscosity of the oil as it warms up. Hence you could start with a cold 10 weight oil and as you warm it up it get *thinner* - it
*always* gets thinner, but the index improver additive means that it only thins out to the same viscosity as a 30 weight oil would at operating temperature.
Thats a 10W30 oil. It's cold viscosity is like a 10 grade oil would be and it's hot viscosity is like a 30 grade oil would be. The W indicates the Winter grade - a hangover from the days when people changed oil grades as the seasons rolled round as the engines ran hotter or colder. Nowadays with modern liquid cooled engines that's no longer an issue.
Added to this is a series of quality ratings. There are two to consider, the API from the American Petroleum Institute and the ACEA from the Association des Constructeurs Européens de l'Automobile.
API grades are S grades for petrol (Spark ignition) and C grades for diesel (Compression ignition). The current best grade is SL (I think) with SJ still "active" others are obselete.
ACEA has A grades for petrol and B for diesels, so you will see things like A1/B1 for normal use, or A3/B3 for an oil suitable for a modern turbodiesel.
If you look in the Sei's handbook I'll make a guess that they specify a
Not quite. It always gets thinner as it warms up but the magic VI improvers mean it only thins out to how a 40 weight oil *at high tmperture* would be, rather than the 10 weight oil at high temperture.
My handbook says 15W40, although when I asked at FIAT they assured me that it was 10W40 even after I asked him whether he was sure. I used
15W40 at first, but I changed to 10W40 mainly because I could get it MUCH cheaper, and I don't think it will kill my engine.
Most other Seicento owners (mostly with the 1.1 engine though) use
10W40 too, but some have been using 0W40 (fully synth). One of the guys with 0W40 says that his engine's still going well after 76k miles. 0W40 can leak on Sei's though so I'm gonna leave that alone.
One thing that seems to happen though is that the 10W40 leaks as it gets to the end of its life. I found another Seicento owner who said that and I experienced a loss of oil when my oil was about 5 months old. If oil does leak like this is it likely to open things up, and make the leaks permanent/worse?
The other thing I still don't really understand is how thin oils (eg.
0W40) leak. They should never get thinner than 40 weight oil. Or could it be due to the oil being like say a 20 weight oil at say 70C, and being a bit thinner than 40 weight oil at 100C?
Interesting fact : 0W40 is actually no thinner tha 5W40, it just has a lower pour point. This basically means that it is suitable for stupidly cold temperatures of something like -30C. However in these temperatures the oil is going to be the least of your worries!
It won't assuming you change it at the required interval. It'll probably give a better cold wear performance as well.
I should hope so - any angine with moderate care should do considerably more than that.
Sure it's not burning it instead? It's only leaking if you can see a puddle forming.
There are several oils where the 10W has a lower pour point than the 5W oil. Pour point is not related to viscocisty - water has a viscosity much lower than most oils for example but it's pour point is 32F. Pour point is related to the chemistry of the oil - a mineral oil will generally have amuch higher pour point than a synthetic yet they will have the same weight and viscosity measurements.
It's possible, actually it probably is being burnt. I suspect it's because it's getting through smaller gaps then it did when it was fresh. My head's oily anyway, so I can't really be sure about that, but I'm gonna clean it up when I get chance.
All engines will use oil to some extent so unless it's excessive then I wouldn't worry in the slightest. A pint or so over the service interval is nothing to worry about unless it suddenly starts.
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