I can’t remember which I used to fill the car with last time. If I put the other type in, there shouldn’t be a significant problem should there? Also, can someone remind me about the difference between the two? I’m pretty sure it has to do with viscosity at different temperatures/seasons. Why such a system that is not intuitive to the uneducated driver, usually maintenance info is all too clearly spelled out (sometimes even on the dipstick itself, like my ATF fluid)? www.oilanalysis.com was apparently “beyond” such basic questions, I gave their site little attention.
It’s not a problem. Go ahead and mix them.
40 has more viscosity than 30.
These sorts of threads tend to go on for a page or so as people have differences of opinion on this. My opinion as an ex-mechanic and current ME is that if your car needs 10W-30, adding a quart of 10W-40 will do absolutely no harm at all. If it truly needs 10W-40, 10W-30 might not provide the full range of coverage needed for high-speed hot-engine driving, but in reality, a single quart when mixed with the balance of your oil will do no effective harm to the vehicle.
The system is not “intuitive to the average driver” because some things in life are just difficult. And multi-viscosity oils are, at heart, not intuitive. But then again, the average driver is only vaguely aware of how their engine works in the first place. In that case, the average driver might note on a scrap of paper or a log book what oil they filled the car with last time…
It is not spelled out on the dipstick because, for many cars, different climates yield different recommendations for the oil weight range to use. A car driven in Arizona in the Summer may very well need a different range than one driven in Canada in the Winter, for example. The owners manual of the car will provide more details.
It’s not all that un-intuitive, once you figure out what they are talking about. The numbers are just a viscosity rating. You can buy 30 or 40 weight oil, and you can also get multiple viscosity oil such as 10W-30 or 10W-40. For a multiple viscosity oil, the first number is the viscosity when it is cold and the second number is the viscosity when the engine has reached its operating temperature.
As Anthracite mentioned, the chart of recommended oil weight vs. environmental temperature range is too big to fit on your dip stick.
Depending on where you live, it might even be recommended that you put one type of oil in during the winter and another in summer. Usually you can find one weight that works for all seasons though.
Turn that around and it is mostly correct. Oil thins as it is heated and the viscosity is reduced. There is more info here.
No, leave it un-turned around and it is exactly correct. The “W” stands for “winter,” and indicates a rating derived from a low-temperature test. The number without the W indicates a rating derived from a high-temperature test.
The whole point of multi-viscosity oil is that it flows well cold and protects well warm. Among the various additives that facilitate this are some that minimize the “thinner when hot” characteristic of plain oil. More info here: http://www.vtr.org/maintain/oil-overview.html
All the previous responses were dead-on.
Well, when I’m going out drinking, I always try to remember:
10W-40 before -30, never sicker,
10W-30 before -40, in the clear.
On a related note, say you run 10-40 weight in your car. So it has a diferent viscosity at different temperatures. Is the fill line on your dipstick more accurate before you run your engine or after it has reached its normal operating temperature?
Or does it not vary that much between those two? I’ve always wondered this but never bothered to compare.
What about synthetic oils? If I were to add a quart of synthetic to the rest of non-synthetic, would there be issues? What about the other way around (non-synthetic into synthetic)?
Is it correct to assume, then, that if your car calls for 10W-40 in summer and you use 0W-40, all is okay because the oil’s viscosity at high temperature will protect the engine from wear. In other words, perhaps the low viscosity is good for start up, but the higher end (40 weight) will provide needed protection at hot temperature.
What am I missing here?
I wouldn’t assume that, because the 0W properties might not be sufficient in place of the 10W that’s called for. Even in summer temperatures, an engine is considered cold until it reaches its normal operating temperature. When the engine is warmed up, the 40 rating would be relevant and would be fine. The concern here is wear during start-up and warm-up.
That said, the trend over the last 20 or so years has been to lower W numbers. 5W30 is the most commonly recommended viscosity for modern cars, and Ford even issued a bulletin retroactively specifying 5W30 for older engines that originally called for 10W30 or 10W40. Honda calls for 0W20 in some of its latest models. My take on this is that research has indicated that good flow is more important than film strength in protecting a cold engine, but I’m not a tribologist so don’t quote me. 
Not a problem. Some companies sell sythetic/conventional oil blends. You can make your own blend, which will cost less money and more time. The higher the proportion of sythetic, the more of sythetic’s benefits realized.
I don’t believe engine temperature makes a noticeable difference. There will be a noticeable, if small, difference between reading the dipstick after the engine has sat for several hours compared to reading it shortly after it has been shut off. This is caused by continued drainage of oil from the upper parts of the engine into the crankcase once it stops running.
Dipsticks are not precision instruments, and a few ounces over or under nominal “full” is not significant, so accuracy is not the issue. For consistency, check the dipstick one minute after shutting off the engine.
Yeah, unfortunately though, some mix this up into “10W-30 before -40, never sicker, 10W-40 before -30, in the clear.” It really makes it confusing when they do that. 