When looking at the viscositys, what does the various parts mean? Is the 10 in 10w30 the weight of the oil after it heats up or cools down? What about the 30 and the "w"? Also, why doesn't SAE 30 or SAE40 have the second number? Does that mean they don't change viscosity when they heat up? I was just thinking (EVERYONE watch out!) about it and I realized I had no clue. The seach function also was a tad unhelpful.
The 10 is the "weight" of the oil when it is cold. This helps out on cold starts. Lower viscosity oils flow better. The W means winter. This oil is rated to be used in the winter time. The SAE sets the standard as to how much an oil flows at a given cold temp. If and oil meets this standard it considered a winter rated oil. The 30 means this is the "weight" of the oil when it is hot. Again, the SAE set the standard as to what is hot. What this all means is some petroleum engineers took a 10 weight base oil and sprinkled in some very small coiled polymers (AKA plastic). When the oil is cold the little coils of plastic are coiled up and short (lower viscosity oil have shorter molecular chains). So the base 10 weight oil is still a 10 weight oil, more or less. As the oil warms the coils unwind and become longer and effectively simulating a heaver weight oil (heaver weight oils have long molecular chains), hence the 30 weight oil.
SAE30 and SAE40 oils are straight weight oils. No polymers are added to make then multi-viscosity oils like 10W-30. They stay at the same viscosity rating over the operating range of the oil.
Like the other guy said, just the oil viscosity ratings for cold and warm temperatures.
What this means for practical purposes is that for modern engines, in general a dino based 5W30 oil is good for below freezing to maybe 100 degrees, a little thin for my taste above 100. A 20W50 would be good for maybe 40 degrees to well over 100, turns to fudge below freezing. The range varies a lot depending on engine condition and how you drive it. An old loose engine will want heavier oil. Towing in hot conditions would be safer with heavy oil. If you live in Alaska and it is winter, you might want to go with a 0W30 oil.
Synthetic flows a lot easier and the viscosity almost doesn't matter. A 5W30 syn should be good for almost anything you can throw at it. The 0W30 and 20W50 syn are more marketing in my opinion. I run a 5W30 syn in all my vehicles. This includes high performance, towing in extreme heat and cold, etc. No complaints in almost 25 years.
Dino is excellent too, but you need to be more selective in viscosity choice.
Don't know if the following argument is still valid since dino oils are much better now, but 5 or more years back there was a big problem with the wide spread range of viscosity. Anything wider than about 20 points was considered risky. For example 10W40 was very popular, but the 30 point spread meant it had a lot of viscosity index improvers, essentially plastic. Plastic is a poor lubricant. The 10W40 oils were causing problems in stressed engines where essentially the oil oxidized and sludged things up,it was called the black death, not good. Some auto manufacturers voided warranties if you used 10W40. Interestingly, 20W50 did not have the problem since it startred with a heavier base oil(20W), so it did not oxidize like the 10W base oils did. None of this applies to dino and perhaps with new technology that is moot. Still there are enough choices that I would just avoid 10W40 anyway.
Just my opinion and I am only a user, not an expert on oil.
Last edited by jim henderson; 11-27-2003 at 10:06 AM.
Here is what the definition of a multi-viscocity oil.
"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-."
Oil does not thicken when hot. It is not like the gravy y'all just ate on Turkey Day. It resists thinning.
The key thing to remember is that a viscosity number means nothing with a temperature at which the viscosity was measured.
In the case of 10W30
the 10 is the viscosity as measured at 0 deg C (32F)
the 30 is the viscosity as measured at 100 deg C (212F)
If you look at a UOA you will see that the viscosity is usually shown in cst at 100 deg C.
In all of the UOA's I've had done, the 10W30 usually return a vsicosity at 100 deg C of about 30-35.
edit: To Jim's comment on 10W40, I would not use it. With the lighter base stock there's more VI improvers (polymers) needed. These polymers do not provide lubrication - they simply keep the viscosity of the lubricant (oil) higher at temp. There's more to it, but there's no reason to use 10W40 with all the other options out there.
The way I think of it is 10W-30 is a 10 weight oil that acts like a 30 weight oil when hot. The 'W' refers to the oils ability at a wide range to temps and can be used under more sever conditions than a SAE 10 oil.
For example, SAE 10 would be recommended for cold weather duty while SAE 30 would be used for warm temp duty. 10W-30 can cover both.
The SAE grades (5, 10, 20, 30, 40, etc.) are categories. I did not see the viscosity table for these grades in the referenced links, but may have missed one. However, the viscosity grades are very nicely laid out in the table at this link:
If you look at the 4th and 5th columns you will get a range of 100 C viscosities for each SAE category. So not all 10W's are the same, nor are all 30's. As a point of reference, water has a viscosity of about 1. Also, in the far right column is the high sheer viscosity, which tells you about viscosity performance under pressure in your main bearings. Notice how the base oil makes a difference here as the 15w40 has a higher high sheer viscosity than does a 10w40.
remember that most friction and wear occurs to an engine during initial start-up when it has been sitting and is cold. All the excess oil has drained back into the crankcase. So you want to get lubrication to the upper parts as soon as possible. How many times have you seen someone start the car even in very cold temperatures and take off without letting it warm up some ???
I would rather have a vehicle with higher highway miles than one that is stop and start all the time. I would use what the engine manufacturer recommends for your particular weather conditions.