Multi weights

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I understand the reason for multiweight oils. My question is this. Using a 5w30 oil as the example. Is it a 30 weight, with additives to make it flow like a 5weight when cold, or is it a 5 weight, with additives to make it protect like a 30 weight when hot ? Or , is it a say 15 weight, that has additives that take it in both directions ? I realize the answer may not be as simple as I have presented it here, but just for the layman, I am curious.
 
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As I understand it, it is basically the last scenario you outline, but there are many variations. Some oils may be a '15 or 20' weight base oil, with VII's added to make it to the 30-weight, and PPD's to make it down to the 5-weight part. I think that was the 'older' way it was done. I suspect a lot of SM/GF-4 5W-30's are actually a mix of base oils - a thicker, group II or II+ base, combined with a lighter group III 'component' to give it the 5-weight part. there will also be some VII'd added to make the full 'stretch'
 
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Keep in mind that a base oil that's a [dino] group II or II+ can have a VII before any additives in the 110 to 120 range. Most 10w-30's are only about 140. So it's not far to go to get there either by adding higer VII grpIII's [highly refined dino often referred to as synthetic] or with either method in the original post. You still see SAE 30 oils around, but assuming they were based on grpII only base stocks, they could probably pass the cold flow tests to be a 20w-30 or 15w-30 pretty easily, with no VI additives. So it doesn't start with a 5 weight with additives, or a 30 weight with additives. It starts with something like a 15w-30, and likely pour point depressants are added. A synthetic base stock could have a good enough viscosity index not to need any, though who knows how often this happens.
 
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travlnman

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Thanks for the input. I had assumed both scenarios were possible. I was also assuming that it probably varied by the manufacturer as well, just wanted to be sure, you know what happens when we assume :)
 
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Trav You're starting off on the wrong foot. There is no such thing as a 5 wt oil. There IS a 5W wt oil. What's the difference? The 5W oil must have a viscosity at -30°C no greater than 6600 millipascals. It must have a viscosity at 100°C no less than 3.8 centiStokes. A 30 wt oil has no cold viscosity spec. Its 100°C viscosity must be between 9.3 and 12.49 cSt. Here is the SAE J300 viscosity spec chart: http://www.infineum.com/information/api_...%20-%202008.pdf So...so that means that the oil maker has a choice of base oils with different viscosity index values and a choice of how much pour point depressant and viscosity index improver to add. All oils (with the possible exception of certain full synthetic oils) will have both of these additives in them. The quality of the oil, especially the quantity and quality of the VIIs in the oil to make it test thicker when hot, is up to the desire of the oil maker to have a better product or a cheaper product.
 
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