Bill.
Spot on - all explained in 'Oil specification and viscosity information' at the head of 'Technical matters'.
All oils thin (reduce viscosity) with increasing temperature. Multigrades are so called because they contain additives which reduce the rate of thinning. A 10w/40, say, has the viscosity relating to an SAE 10 when cold, but when hot is equivalent to a 'hot' SAE 40. Thus they give the cold start protection of a low viscosity oil, but provide the hot protection of a higher viscosity oil.
JS
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Simplistically a straight, for instance, SAE30 oil is a lot thinner when hot than when cold though a 10W40 is still thinner when hot though the difference is a lot less.
Of course it also depends on the definition of hot and cold, the relative temperatures.
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cheddar
Yes, any oil is thinner hot than cold. However, a 'straight' SAE 30 would be more viscous (thicker) than a 10w/40 when cold (because the 10W/40 has an SAE 10 viscosity when cold) but less viscous than the 10W/40 when hot (because the 10W/40 is acting as a hot SAE40).
JS
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Ok, maybe my example was not the best, I take your point, what I meant was the difference between viscosities hot to cold is less on a multigrade than a straight oil. To use quote your example ...
Yes, any oil is thinner hot than cold. However, a 'straight' SAE 30 would be more viscous (thicker) than a 10w/40 when cold (because the 10W/40 has an SAE 10 viscosity when cold) but less viscous than the 10W/40 when hot (because the 10W/40 is acting as a hot SAE40).>>
... a hot 40 and cold 10 are likely to be similar in viscosity when a cold 30 and hot 30 will be different, that is the whole point of multigrade oils.
Regards.
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I'm even more confused now, there seems to be a contradiction here. The majority view seems to be that the oil is thinner when hot, but if a 5W/40 oil is supposed to be 5W viscosity when cold and 40 when hot then surely it is thinner (less viscous) when it is cold, so will flow better, or am I missing a point.
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I'm even more confused now, there seems to be a contradiction here. The majority view seems to be that the oil is thinner when hot, but if a 5W/40 oil is supposed to be 5W viscosity when cold and 40 when hot then surely it is thinner (less viscous) when it is cold, so will flow better, or am I missing a point.
That's why I didn't try to explain! See 'oil viscosity' here:
minimopar.knizefamily.net/oil.html
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Here's an explanation as to 'why':
From: tinyurl.com/djzfg
'Multi viscosity oils work like this: Polymers are added to a light base (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.'
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These American websites are a bit odd, they recommend that with ordinary oil you change it at 3000 miles, and with synthetic every 5000 miles!
No wonder there is an oil shortage.
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So why do they have such huge engines out there then? I always thought it was because of the miles they do (Not really as much of an issue nowadays - look at the mileage of US cars on eBay!).
But any car getting it's oil changed every 3k would go round the clock twice, easily! And with more miles to the gallon, to boot, if it had a smaller engine.
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So it's a good idea to change the oil at 3000 mile intervals, is it?
I've always been keen to change my vehical's oil regulary, but I think that every 3000 miles is a bit excessive.
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So it's a good idea to change the oil at 3000 mile intervals, is it? I've always been keen to change my vehical's oil regulary, but I think that every 3000 miles is a bit excessive.
Oil's a lot cheaper in the US - In Walmarts, Mobil-1 fully synth was around $20 and that was for 5 US Quarts (4.73L). 'Ordinary' oil was just a few dollars.
There does seem to be sort of culture of frequent oil changes in the US - not sure why that is?
I've certainly seen comments from mechanic's saying they change oil very frequently, but other people say it's pointless.
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It depends on the kind of mileage you do. If it's around town with lots of stopping and starting, I always change my oil every 3K miles max. If it's up and down motorways without excessive holdups, then 5-6K is fine. To underline the fact that driving conditions dictate frequency, I changed my oil monthly when driving in the middle east.
The engine will definitely appreciate some tlc.
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New clean oil is a pale golden colour, depending on make, and very transparent. In use it'll gradually darken to black and become totally opaque - this is the time to change it.
Short stop/start runs will blacken it much, much quicker than very long runs.
Let the appearance be your guide.
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New clean oil is a pale golden colour, depending on make, and very transparent. In use it'll gradually darken to black and become totally opaque - this is the time to change it.
Except with a diesel
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Please correct me if I'm wrong, but someone told me a few years ago that oil for diesel engines i.e. Castrol GTD contains a detergent which cleans the oil when driving continually for a long distance. Hence it is said that cars which are subject to a lot of stop/start short distance driving should have more frequent oil changes. I normally change the oil in my Peugeot 306 1.9TD about every 5-6k but now I'm doing a bit more town driving than I was, I'll probably change it more frequently.
Having been driving Peugeot XUD diesels for the last 5 years, I know that within a few hundred miles of changing the oil, the new oil is black- even though I nearly always use Castrol GTD
Regards
Martin
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contains a detergent which cleans the oil when driving continually for a long distance.
Close, but not quite :-)
Oil for diesel engines contain more detergents than those for petrol engined cars. This is because diesels produce more combustion deposits like soot, so in order to stop sludge build up, detergents are added to clean your engines internals.
Mind you, I could be wrong there, but that?s what I understood to the main difference between petrol engine oil and diesel engine oil.
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Interesting! I've just bought my first ever diesel, (Yaris 1.4 D-4D) and I'm very pleased. The Dealer changed the oil at the time, but some 500 miles later, the oil is quite black. Why should there be combustion products in the oil anyway? Surely, the bottom end of the engine is much the same as a petrol version and the oil has much the same routing as in a petrol version.
I thought these days with catalysors fitted, crankcase breathing was very controlled to keep the emissions down. I would be grateful for a more detailed explanation if possible.
Regards,
Jaytee
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Why should there be combustion products in the oil anyway?
As with all engines, when the air/fuel mixture is compressed and ignited, some of the gasses and fumes will leak past the piston rings. It?s almost impossible to make each combustion chamber 100% sealed, especially when compression pressures are in the region of 10-15 bar (double that for diesels). This is otherwise knows as 'blow by'.
>>Surely,the bottom end of the engine is much the same as a petrol version and the oil has much the same routing as in a petrol version.
Yeah, that?s correct, but those blow-by gasses or fumes will contain particles like soot which would collect on the pistons, piston rings and oil galleries. Bearing in mind that the combustion of diesel fuel generally produces more soot that petrol (hence your oil turns black quickly). It is therefore essential that oil for diesel engines contains enough detergents to both clean those deposits from the engines components and keep them in suspension without clogging up the oil ways. I thought these days with catalysors fitted, crankcase breathing was very controlled to keep the emissions down.
Yes, crankcase emissions are tightly regulated for that very reason, but it has nothing to do with the oils (if I understand your questions)
Hope that helps, and likewise, if I?m wrong (likely), then please correct what?s wrong.
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