Since I changed the oil and filter on our 05 multijet at 31k miles using Castrol fully synthetic oil, I have noticed the fuel consumption improve from around 54 to just over 58 mpg on the trip computer.Engine also seems smoother, though performance seems no different. These figures might seem low but car is used for short city journeys or fast motorway trips.Anyone else notice anything like this?

I've just put mobil1 fully synthetic 0w40 into my wifes espace.
mpg has got better too. I'm sure I read that they reckon you can save fuel as the oil is "more slippy" so the engine turns easier.

correct me if i'm wrong

The thinner oil will provide less resistance (so less power loss/more economy), but there are downsides, such as it can pass piston rings more easily.

Have used full synthetic from millers and Mobil in my past cars and got improved MPG, quieter running and less smoke, so it does not surprise me.

Also do bear in mind that car manufacturers quote viscosity figures for a reason, including that mentioned by H. above.

Going from 5w40 or 10w40 to 0w40 (all fully synthetic) absolutely makes sense. It will not harm your engine. In fact, the only difference will be reduced wear on cold starts.

The 40 in the oil weights mentioned is the SAE rating at operating temperature. Since it's the same for all three, they will be exactly the same viscosity as soon as the engine is warm.

The number before the w however, represents the SAE rating when cold. This is not directly the same scale as the number after the w, though!

This means that when the engine is cold the 5w40 oil will behave as a single-grade SAE 5 oil behaves when cold, but when the engine is warm it will behave as a single-grade SAE 40 oil behaves when warm.

Even 0w40 oil is too thick for the engine to pump properly when cold, but it's much thinner than 5w40 or 10w40 at those temperatures which translates to less wear due to cold starts. At operating temperature, there's no difference.

There is absolutely no reason apart from cost for sticking with 5w40 or 10w40 when 0w40 is available. It's better for your engine.

hxxp://ferrarichat.com/forum/showthread.php?t=136052

Ignore the parts that talk about racing (not exactly applicable for a diesel Panda), but read the rest of it.

Buggered if I'm going to sit for 2 hours and read all that link, but thanks for posting it. One thing I did pick up in the early pages was the mention of not changing oil due to a belief of viscosity curve changes over time, with no mention to the fact it picks up and holds combustion by-product contaminants in suspension and can only hold so much before it begins to abrade the engine internals.

Been around this a few times before, never really had definitive answers. Couple of q's:

Why wouldn't manufacturers state a 0w40 / 0w30 if it was 'better'?

What viscosity oil do you need for gap filling and film strength properties to prevent metal-metal contact in your Diesel engine when cold, before it's all warmed up?

HP. said:

Why wouldn't manufacturers state a 0w40 / 0w30 if it was 'better'?

What viscosity oil do you need for gap filling and film strength properties to prevent metal-metal contact in your Diesel engine when cold, before it's all warmed up?

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Manufacturer recommendations are based on what the engine was originally designed for. The engine will definitely work with the recommended oil, but better alternatives may have surfaced in time since the engine was designed.

As an example, the 1.4 100hp engine is an older design than the Multijet 1.3 engine, which is why the former is specced for 10w40 semi-synth and the latter for 5w40 full-synth, simply because of what was available and economical at the time.

Any modern engine will benefit from a change to fully synthetic with the cold SAE rating as low as possible and the operating temp SAE as the manual specifies.

Also, keep in mind that even 0w40 is much thicker when cold than even a 20w60 oil is at operating temperature, which is why 0w40 is much more than sufficiantly thick at cold. But it's thinner than the more commonly used 5w40 and 10w40 when cold, which gets you to the proper oil pressure sooner.

I'm sorry but I remain sceptical. What evidence is there that it's thick enough when cold or has the required gap filling and film strength? By this reckoning, there would only be need to make one grade of oil.

What warranty implications are there when running beyond manufacturer's recommended grades?

As you can see from this graph from the link I posted, 0w30 is much thicker when cold than when at operating temperature (212F/100C):

Synthetic oil:

Oil type...Thickness at 75 F... at 212 F...at 302 F

Straight 30...........100...................10...........3
10W-30.................75...................10........ ...3
0W-30...................40...................10...... .....3

Meaning, that if it's thick enough at operating temperature, it will definitely be thick enough when cold. Which means more gap filling and film strength when cold than when warm.

In any case, this doesn't really matter since as soon as proper oil pressure has been created and the oil pressure light goes out everything is a-ok. With a 0wXX oil, this will happen much faster.

I'm sorry but you haven't really answered the question at all.
You're spot on in what you say about the oil viscosity changing when hotter, no argument.
How do you know it's definitely thick enough when cold, when you're suggesting to people they go one or two grades lower on the cold viscosity rating than the manufacturers recommend? After all, they designed the engines.

How can you say it doesn't really matter when we don't have evidence that the oil is the correct thickness to provide mechanical protection when cold?
Agreed a thinner oil will flow faster, but we don't have evidence it's providing better lubrication.

If it was fine to use a 0w40 on an engine specified by Fiat to be run on 10w40, why wouldn't Fiat suggest this as a premium oil for the 'better protection' that people keep talking about?

In fact, why would they bother to reccommend different grades of oil for different climates - surely just tell us to use a 0w40 wherever we are?

I know it's thick enough when cold because even a 0w40 is much thicker when cold than it is when warm. Therefore, it's definitely thick enough, and most likely even too thick when cold.

It's all pretty thoroughly explained in the link I posted.

...so why don't all manufacturers recommend a 0wxx oil for their cars as a premium option?

If a thinner oil is better, why aren't we using 0w20 or 0w10 oils?

I really don't think it's as simple as you believe. In that thread the author is mostly talking about his Ferrrari and swapping from a 5w oil to a 0w - you can't simply apply the same principles to all Fiat Pandas and recommend we all run 0w oils.

Some quotes from the thread you're referring to:

"........ Porsche specifically prohibits a 0W-XX engine oil....."

"Automotive engine manufacturers know these principals of motor oils. They know there is thinning or thickening that will occur. They take these things into account when they write that owners manual."

A thinner oil is not necessarily better, but I'm not saying it is.

What I'm saying is that going with an oil which is thinner when it's cold is better for the engine and causes less cold wear. It's still much thicker than the "proper" oil is at operating temperature, so you won't ever run into problems with it being too thin.

If an engine is designed for an Xw40 oil, it most likely needs an oil which behaves like an SAE 40 oil at operating temperature. That's why I'm not telling you to put 0w20 or 0w10 oil in your Panda. It's too thin at operating temperature.

Also, you can't take the guidelines for a 6 or 8 cylinder high-performance Porsche engine and apply them to a 4 cylinder diesel engine made for economy. Porsches will probably be driven quite hard and get much hotter than a small diesel engine will ever get. And different engineers have different philosophies wrt. lubrication, wear, heat and so on.

What I'm saying is that you can safely swap the recommended 5w40 to 0w40 in your Panda. It will change absolutely nothing when the engine is warm, but it will help prevent damage from cold starts. It will absolutely positively NOT be too thin to lubricate properly.

Yes, you're quite right about the oil being thinner when hot - no denying that.

But still, if the manufacturer states a 10w40 oil for my car, it needs to be the 40 when hot. Can you explain why the 40 hot viscosity is important - then explain why the 10 cold viscosity isn't?
You have stated a 0w40 oil will be better for mine.
I ask again, what evidence do you have that a 0w provides sufficient cold start protection (apart from the fact it will circulate marginally quicker from a cold start) and is ok if the manufacturers state 10w?
Is the gap filling and film strength sufficient?
Isn't the engine designed to cope with a 10w on a cold start? This is why manufacturers state a lower w figure for cold climates.

Yes, a thinner oil will circulate very slightly faster from a cold start, but there are other important properties to lubrication.

HP. said:

But still, if the manufacturer states a 10w40 oil for my car, it needs to be the 40 when hot. Can you explain why the 40 hot viscosity is important - then explain why the 10 cold viscosity isn't?

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The two numbers should not be considered using the same criteria. The value after the w is for oil viscosity at operating temperature which is the state your engine is hopefully going to be in for as near as possible its entire operating life.

The value before the w only applies when the engine is started from cold (or more precisely from ambient temperature). Even though the value is so low, it's still quite a bit thicker when cold than the 5w40 probably in your engine right now is when at operating temperature.

HP. said:

You have stated a 0w40 oil will be better for mine.
I ask again, what evidence do you have that a 0w provides sufficient cold start protection (apart from the fact it will circulate marginally quicker from a cold start) and is ok if the manufacturers state 10w?
Is the gap filling and film strength sufficient?
Isn't the engine designed to cope with a 10w on a cold start? This is why manufacturers state a lower w figure for cold climates.

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Yes, the gap filling, film strength and viscosity are sufficient.

0w40 is closer when cold to the SAE Xw40 oil Fiat recommends for the engine than 5w40 is when cold. At operating temperature, they're indistinguishable, but when cold, the 0w40 is closer to providing the correct lubrication properties the engine was designed for.

HP. said:

Yes, a thinner oil will circulate very slightly faster from a cold start, but there are other important properties to lubrication.

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I don't consider a possible 25% oil flow improvement or more on cold engine starts something to be dismissed as being "very slightly" better. That flow improvement could easily add 50.000km to the life of your engine.

Which is why for modern synthetic oils, you want the value before the w as low as possible and the value after the w should be what the manufacturer recommends.

Why don't Fiat recommend it then?

edit: Here's a website that confirms my suspicions that too thin an oil can cause bearing issues:

http://www.blackstone-labs.com/oil_viscosity.html

As the thread starter I must point out that the oil I used was 5w40 ie what fiat specify anyway I think if your vehicle is fairly recent and running well it is not worth trying any other Surely they wouldnt make so many different grades if they werent needed I was quite bewildered when I went into halfords

HP. said:

Why don't Fiat recommend it then?

edit: Here's a website that confirms my suspicions that too thin an oil can cause bearing issues:

http://www.blackstone-labs.com/oil_viscosity.html

Click to expand...

They likely recommend 5w40 for a number of reasons. Perhaps there was no Selenia (or other brand) 0w40 widely or at all available at the time the engine was designed. Or perhaps the better grades of oil were deemed too expensive (the Panda being an economy-minded sort of car).

Blackstone labs definitely know what they're talking about. but in this case they're talking about viscosity at operating temperature, where you should definitely stick to the manufacturer recommendation, unless told otherwise by a highly experienced mechanic.

This doesn't change the fact however, that 0w40 is thicker when cold than 5w40 is when warm, meaning that it is definitely not too thin. With both oils cold, 0w40 is much closer to the proper viscosity for the engine, leading to less wear on cold starts.

If you experience a rattle when cold for the first few seconds even with fresh oil, I'd suggest dropping down one grade on the w level but I wouldn't suggest someone change from a 10wxx or 15wxx to a 0wxx oil.