Okay, next oil lesson.
Q: What does a 10W30 and 5W30 have in common?
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bzzzzzzzzzzzz, times up
A: They are BOTH 30 weight motor oils.
#### Warning: Long Winded Post ####
In the realm of synthetics, season has little to do with what viscosity you should run.
Because, as mentioned I am familiar with their products I will use Amsoil 5W30 and 10W30 motor oils to illustrate my point. All this information can be found on their website.
First, lets show that they are both the same thickness.
If you look at the technical properties information for the 5W30 and 10W30 you will see a number for Kinematic Viscosity. One is done at 100*C and one at 40*C. You will see that at 100*C there is no difference and at 40*C it is a negligable difference. Viscosity is a measure of a fluids thickness or resistance to flow, as you can see both motor oils have nearly identical viscosities which would state they have nearly the same resistance to flow which would further state they are nearly identical in thickness.
Okay, moving on, lets look at some cold flow properties to determine which oil is going to be better for cold temperatures. Which one is going to flow less or take longer to flow in cold or extrememe temperatures. If you again look at the technical properties sheet for both products you will see a number for Pour Point. This is a measure at what temperature the motor oil will cease to pour. In other words, at temperature at which point the oil would have to reach to virtually cease flowing. See the number? Its -58*F. Anyone around here live in that climate? If you do, you need other options. Perhaps a 0W motor oil or a block heater.
So you may be asking, then, what is the difference between the two? Well there are two things to look at:
1. Cold Cranking
2. Evaporation rate
First the Cold Cranking.
Referring back to the technical properties sheet for both products, you will see a listing for CCS Viscosity. This stands for Cold Cranking Simulation. In technical terms the Cold Crank Simulator Test determines the apparent viscosity of lubricants at low temperatures and high shear rates. In laymen terms it is a mesure that directly relates engine cranking and startability under cold weather conditions.
The problem here is that you cannot make a direct apples to apples comparison. The reason is that it is stipulated that 10W30 motor oils be tested at a different temperature than 5W30 motor oils. 5W30's are tested at -30*C and 10W30's are tested at -25*C and that small of a temperature range does make a difference. If the 5W30 were tested at -25*C its CCS cP (Centipose) would likely be in the mid 3000 range. Just guestimating, I dont know for fact. The pont being is that a 5W30 will offer less resistance to starting than a 10W30.
That know, consider this. How many of you start vehicles in that low of a temperature? Again, if you do, you are likely equipped with a block heater or you need to consider something along the line of a 0W motor oil. I doubt many would be able to tell the difference with the exception of those in the extreme north east, north central areas and Alaska. And consider this, how much time do you spend starting your vehicle? If your battery is in good state and the starting system in good shape a 5W30 or 10W30 is going to make little difference.
Next is the Evaporation rate, and this is where the diffence between the two is key. Looking back at the technical properties sheet, you will see a line that shows Noack Volatility. This is a measure of a motor oils evaporation rate. And is measure in the % of weight lost. Want to guess what time of year this really applies? Nope, not winter. Notice the 5W30 has higher evaporation rate than the 10W30 does. Odd huh?
Ever wonder where your oil goes between oil changes and you know you filled it up to the full line, but come time to change the oil and it looks like you are a 1/4 qt or 1/2 quart low? You got no external leaks, no burnt oil smell nothing on the ground. Well, its likely you have lost it to evaporation. Want to know what the failure point is for petroleum motor oil? 15%!! Yep, if a petroleum motor oil evaporates more than 15% it fails API certification. What does THAT tell you about API standards?
Essentially a 10W30 motor oil is more shear stable than a 5W30. So if I have a customer that comes to me and says I have checked everything, my mechanic has checked everything and no where is a leak found but I keep loosing this oil, I'll first put them on a 10W30. In many of the cases using a 10W30 has significantly reduced or completely eliminated the oil loss. If it continues then I get into testing the motor oil to see if there is some sort of internal issue.
If you look at the rest of the technical sheet you will see that all other tests run show nearly identical numbers. TBN, HT/HS, Four ball Wear test and so on.
So with regards to which oil you should run in the winter or summer, it really does not make a difference in any case but really extreme ones. You'll be fine with either one. I know I could have said that in the beginning, but aren't you glad you read all this now? Dont you feel just smarter? Think of all the great trivia you just learned to impress your friends with.
Shameful I know, but I'm part of the old timer crowd.
Guessing doesn't seem to be a good idea. Heck the GM stuff isn't much $$ for how long it can be left in.