Oil viscosity?

What's the deal with oil viscosity? for example, 10W30 as opposed to 10W40? i know that the W means use it in winter. but whats the difference between the series of numbers?Oil viscosity?
yes the W does stand for Winter. The numbers are the viscocity (thickness) of the oil. The smaller the number the thinner the oil. So a 10w30 would start out as a 10 weight cold and when the engine warms up it would be a 30 weight. The 10w40 would start out at a 10 weight and turn into a 40 weight. Some people use SAE 30 oil which is a 30 weight all the time but that is hard on you car in the cold and for start up. The smaller the number in the front the faster it get to the top of the engine. Which is the most important thing to get lubricated at startup. that is why most newer cars are using 5w30 or 5w20 oil. Some even use 0w20 oil now. It is really about a 2 weight but they round every number to the nearest 5 up or down.Oil viscosity?
Viscosity is the flow-ability of an oil. 10W40 for example is actually 10wight oil to which viscosity modifiers are added so it will maintain the SAME flow-ability over a large temperature range,, for example 10w flows at a given rate at 0C and 40W will flow,,, in theory.. at a higher temp like the 10W flows at 0C if you use the oil over a LONG period of time,, the viscosity modifiers are destroyed and you will be left with 10W oil. The numbers 10W, 30W, etc are arbitrary numbers assigned to oil by ,, if I'm not mistaken either the Society of Automotive Engineers or the Petroleum Institute.
Oil viscosity is the thickness. Not the flowability. I dont even think thats a word. An oil with a thicker viscosity will be more able to keep the metal components in your engine separated. Creating a cushion of sorts beetween them. A thinner viscosity oil will flow more freely and be able to cool the metal parts better.





A multi Viscosity oil. IE 10w30. Flows like a 10wt and creates a cushion like a 30wt. So on and so forth with all multi viscosity oils.





As for the W. It is there to label the two numbers. It is the weight or viscosity of the oil. They put it between them to separate them and it applies to both #'s.





Hope this clears up any confusion.
It's multi-viscosity.Thinner when cold and thickens as it warms up.
the first number is viscosity and the last number is the weight.
Actually, I don' t think the W means winter, but I can tell you what the numbers mean.





The numbers indicate the weight or relative viscosity or flowability of the oil. A 10W40 oil acts like a 10 weight oil until it reaches operating temp, and them it performs like a 40 weight oil.


10W oil is thinner than 40W oil.





A 5W30 flows better in cold weather than a 10W30 oil, but they both flow the same way after the engine heats up. A 20W50 oil is thicker than a 5W30 in both cold weather and after it heats up.





Hope this helps.
10w40 for example flows like 10 weight when cold and like 40 weight when hot. It is called multi viscosity oil. You see the advantage in this is that most oils flow slower when cold and faster when hot ,just the opposite of what is desireable. Viscosity is a measure of the ease of which a fluid flows. If multi weight oil is actually thinner when cold and thicker when hot I do not know.
it's a multi-viscosity oil. 10 represents the oils viscosity when it's cold. This allows it to be thin for better performance at start up. 30 or 40 is the viscosity when it warms up so it doesn't get too thin. both the 10w-30 and 10w-40 oils act as a 10 weight when cold, but the 40 is thicker when warm.
the w means weight. The 10 is the weight of the oil when cold (thinner) and when it gets hot it is 30 (thicker) they are just trying to have a good oil over different driving temps. and conditions.