I'm a little confused about viscosity numbers...

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Dec 31, 1969
I guess it's finally time for me to get this straightened out. I know when to use what viscosity, but I'm a little confused about what the numbers mean. The first number is cold right? And the higher the number the thicker it is? If that's true...wouldn't that mean 10w30 gets thicker as it heats up? I know that can't be right... [Confused]
10w-30 means that when it's cold it's a 10 wt oil. When it's hot it's as thin as a 30 wt oil would be at that same temp. So basically it quits thinning out once it gets warmed up....In a perfect world anyway.
Use 10W30 as an example. Looking at the second number first, we find that the 30 means that at 100c (212f) the oil's viscosity is between 9.3 and 12.5 Cst (a measure of viscosity or thickness). 0W30, 5W30 and 10W30 are ALL in this range at this temp. A 20 weight is a little thinner at 100c, a 40 or 50 weight is a little, or a lot thicker, again measured at 100c. (Remember, we are looking at the SECOND! number only) The first number is read 10 winter, NOT WEIGHT! It is an arbitrary value THAT IS IN NO WAY! comparable to the second number. The 10 means that the oil is borderline pumpable at a temperature of -25c (-13f). Comparing other weights, we find that a 0W pumps at -35c (-31f), a 5W pumps at -30c, a 15w pumps at -20c and a 20W pumps at -15c. Oils must pass some other viscosity criteria as well to obtain the various W designations, but in general terms the lower the first number, the lower the temperatures at which the oil can flow and pump. So... if it gets really cold, use a 0w. Kind of cold, a 5w. Not too cold, then a 10w.(or 15w, or 20w) Hope this helps a little... [Big Grin]
You know what? I thought the same thing until I figured it out/researched/asked my mechanic. It almost fits the model to think the w number reflects the "weight" when cold and the other number the viscosity when...hot??? Oil is thinner when hot, my mechanic made light of that, the opposite. It is confusing partly because a lower w number does (mostly) mean a thinner oil (but does not represent viscosity) at cold temps, so most guys probally go on thinking *that* instead of asking, blah, blah. "Cold-cranking rating"? Most guys don't have a clue about what it is and what the w number corresponds to, temp-wise.
Alright...it's slightly more clear now. [Smile] If the first number isn't a weight...why would you NOT want a 5w30 or 0w30? Why is 10w30 better in the summer, for example?
well in alot of circumstances, a 10w30 is more shear stable than a 0w30 because a 10w30 would most likely have less VII. so, its basically a trade off, on one hand a 0w30 will have better cold flow, at the expense of shearing down. on the other hand, a 10w30 will not flow as good cold, however it will be more resistant to sludging. this is where synthetics come into play. a good synthetic may not need any VII at all to be a 0w30. add to that, a synthetic base stock is probably more shear stable than a dino base stock anyways. now, the big question would be how to tell which oils contain VII and which do not. VII isnt really a bad thing though, they help cold flow which is good, but are more prine to shearing than having no VII, it is a case of "to much of a good thing can be bad". just imagine how much VII would be needed in a 0w60 or some wide spread like that.
Why not a 0W or 5W; because it gets increasingly difficult to have an oil that flows when cold AND!!! maintains sufficient viscosity when hot. The goals are not mutually compatible. You will almost never find a 0W30 conventional oil, because the cold weather characteristics necessary to meet the 0W designation do not allow the oil to be a 30 weight when hot. 5W30 is possible with conventional oils, but to do so requires fairly large quantities of viscosity modifiers. These additives prevent the oil from thinning at operating temperature. They do tend to wear out and breakdown; this is where the term "shearing" comes from. The oil (really the viscosity modifiers) shears down, becoming thinner at operating temp., and this can lead to higher consumption and/or deposits. Synthetic oil is, by its chemical composition, better at meeting the cold/hot requirements. A 10W30 synthetic may in fact be a straight weight oil that, through its chemistry, can still meet the cold temperature pumpability and cranking requirements AND! be a 30 weight at 100c. Still, large spreads like the 5W50 do require the same viscosity modifiers to perform at both ends of the temperature spectrum, and shear can be a problem with some of these synthetics as well. What to do...use oil (especially conventional)with the narrowest spread possible; 5w30 and 10W40 can be more likely to breakdown, given the above info. 10W30, 15W40, and 20W50 tend to be more stable for longer periods of time. Synthetics allow for much greater flexibility, but at a much higher price.
Wha? You gotta be kidding me. They CHANGED the system? No wonder most people don't know wtf is going on with their oil specs! Ha ha, how about 20w goes down to 20 degrees, 10w to 10 degrees and so on and so on. A simple rule for simple minds.(non-metric, of course) Whatever, the new system is better, being tested with a load, but kinda crazy to change it. [ December 23, 2003, 03:23 PM: Message edited by: TSoA ]
Originally posted by TSoA: ) Whatever, the new system is better, being tested with a load, but kinda crazy to change it.
It looks to me like they made the system more representative of how oil performs in the real world, which is an improvement. IIRC, they just tweaked the W part of the rating system again recently. It's up to the engine manufactuerer to specify waht oil should be used in their engine, so different engine companies might specify diferent viscosities for the same cold weather temperatures, just like they do for normal to hot weather.
Why don't they just write in big letters on the font of the bottle the temperatures you should use it in?! [Big Grin] [Razz]
Originally posted by ZmOz: Why don't they just write in big letters on the font of the bottle the temperatures you should use it in?! [Big Grin] [Razz]
Because some people would be confused, again, thinking the big numbers might be the "price" of the oil!. No...I'm not serious!
i think they should have posted the actual viscosity as the cold and hot numbers. say a 90w30 = 10w30? did i do that right? anyways, it makes alot more sence that way than this *** backwards 10w30 stuff. i have known some mechanics who think an 10w30 is a 10 weight when cold, and it thickens up to a 30w when hot. i ask them y does oil pressure drop as engine warms up then..... like most things in life, this scale was probably partly designed to be fairly easy to read, however ends up more confusing than it should be. maybe next week we can tackle the topic of octane ratings? [Razz]
The W in 10W-30 stands for Winter. Back when dinasauers roamed the Earth, until around 1970 or 1980 (not sure of exact date), multi viscosity oil was tested and rated at two temperatures. 0F and 212F. 10W meant the oil had the same viscosity as standard SAE 10 weight oil when both oils were tested at 0F. 30 meant it had the same viscosity as a standard SAE 30 weight oil when both were tested at 212F. 10W-30 still becomes less viscous (thinner) as it warms up, it just doesn't get as much thinner as single weight 10 weight oil. It became obvious to tribologists (a strange tribe of people that study lubricants) and automotive engineers that the 0F winter rating wasn't describing true cold weather oil performance. Finally, in 1980 according to one souce I have, SAE revised the W rating system Oil behaves differently depending on how much shear stress it is under and a bunch of other things. So, they revised the W tests to test how well an engine would crank at the actual low temperatures of interest and how well automotive oil pumps could pump the stuff. Whether the engine will crank over at some target temperature is more important than how fast it pours through a simple viscometer at 0F and if it is has too high a low shear viscosity, the oil pump won't be able to suck it out of the oil pan, not a happy circumstance. No automotive oil gets more viscous (thicker) as it gets hotter.
Originally posted by cryptokid: ...maybe next week we can tackle the topic of octane ratings? [Razz]
Don't forget to discuss cetane also...don't want the oil burners to feel left out!
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