My engine says that it takes 10W 30 oil. I accidentally used 10W 40 oil. Should I be worried about this? Is my car going to blow up? LOL. I?m not even sure what these numbers mean.
10 is the cold temp viscosity, 40 is the high temp viscosity. 1 Time will probably not be a problem but a higher viscosity will cause piston ring wear.
Not really. Many of these oils shear down anyway, plus the summer weather makes the issue carry much less risk with it. Some folks consciously use 10W-40 oil in 10W- or 5W-30 applications and their engines haven’t blow up yet, either.
10W means it acts like a 10 weight oil at start up; 40 means it acts like a 40 weight oil at operating temperature.
Are you saying you added one quart of 10w-40 to top it off? If so, no problem. If you changed all of your oil to 10w-40 you’re probably fine for the summer, especially if you have a high mileage car, but I wouldn’t make a habit of it.
Top off with 2 quarts. I wont be doing that again! LOL. Thanks for the info everyone, now I understand the numbers!
this is a suggestion that you should check your oil about twice more frequently than you have been. You should add about one quart at a time. Waiting until two quarts low is not good for any engine.
Thanks Jay, I’ll be sure I do. 
I don’t know how many miles were between the oil change and the 2-quart addition, but I would check it often enough to fill only 1/2 quart at a time. Another thing is that if it’s burning that much oil (enough to burn 2 quarts in less than 3,000 to 5,000 miles), 10W-40 might be the right oil. Check your owner’s manual. It may suggest using 10W-30 for most applications and 10W-40 for extremely hot conditions. If that’s the case, I would use 10W-40 for summer and 10W-30 for winter. Heck, I would even consider using 10W-40 all year around in a place like where I live, where the lowest winter temperature never dips below 25 degrees Farenheit.
Drive on worry free.
Exactly!
MUCH more problematic than the change in oil viscosity is the owner’s apparent habit of allowing the oil level to drop very low before adding oil.
Sarah–You should be checking the oil much more often.
You should never have to add more than 1 qt to the crankcase, and ideally, you would add oil as soon as it is down by 1/2 qt.
How long did it take for the car to burn through 2 qts of oil?
When your engine starts talking to you, you’ve got troubles!
On my 1978 Oldsmobile Cutlass, the owner’s manual states that I may use either 10W-30 or 10W-40 oil. I used 10W-40, but did have problems with spark knock. Every other month I would have to put some Casite Motor Tune-up in the gas tank and then take the car out on the interstate and run it hard to remove the carbon deposits. One Saturday morning on Car Talk, a caller had the same problem. Tom and Ray recommended the person switch from 10W-40 to 10W-30. Apparently, the polymers that give the 10W-40 its viscosity range caused carbon build-up in some engines using some brands of 10W-40 oil. I switched to 10W-30 and the problem of carbon build-up which caused the spark knock disappeared. This Oldsmobile has a carbureted engine. The problem probably wouldn’t happen on a modern fuel injected engines. Your car isn’t going to blow up. (I usually drink BudLite, but nothing bad happens to me if I occasionally drink a regular Budweiser).
“W” stands for “Winter”, not “weight”. 10W means the oil will flow the same at -20?C as a 15W at -15?C, as 20W at -10?C, as a 5W at -25?C. The winter rating is based on the temperature at which the oil reaches a viscosity which is accepted as a good guess as being the thickest your engine oil pump can push through the engine. The winter test is not performed at any one single temperature but represents a temperature.
The number after the W indicates viscosity at 100?C when compared to a same straight weight oil.
As for the viscosity between freezing and boiling one can not make any sweeping statements other than “its somewhere in between.”
The difference between 10W30 and 10W40 isn’t going to harm anything. If you had used a 50 weight or 20 then perhaps there would be reason for concern.