Can you explain engine oil weights?

May 7, 2007 9:49 AM

Can someone explain the importance or relevance of different weight engine oil? How much and why should your local climate affect your oil choice?

[Sorry if this has been posted before...I couldn't find a relevant post.]

I've read that the first number is the viscosity when the engine is cold and the second is the viscosity once the engine is warmed up. Why does viscosity matter?

Does the difference betwen 10Wxx and 5Wxx really only matter in cold climates?

And the difference between xxW40 or xxW30? Does that part not vary according to climate since it should be the same once the car is warmed up?

How does viscosity affect engine life and car performance?

I have a '03 Jetta and the owner's manual recommends 5W40 or 5W30. My VW mechanic (who I trust and respect) recommends a different weight during the summer in Texas (though I can't recall what weight at the moment).

So, if I'm driving my car through the Mexican desert, [how or] should I adjust my engine oil?

If I'm taking a road trip to Michigan in Februrary, [how or] should I adjust my engine oil?

Oh. And I always use synthetic oil...so are these issues different for synthetic vs. regular oil?

posted by mdion to Travel & Transportation (9 answers total)

Lots of substances loose their viscosity when heated (think maple syrup at room tempature vs. after a short trip in the microwave). Since the goal of the oil is to coat parts of the engine to protect it and give it the right amount of lubrication among other things, it is important to have the right type of oil to deal with the type of engine you have as well as the external forces (i.e. temperature of the outside). Too little viscosity and you have lots of metal pieces rubbing against each other. Too much viscosity and the engine spends too much effort in moving its parts thru thick fluid.

As far as to what oil to use when, I was under the impression that most modern car engines and oils are far more forgiving then they used to be but I will let the experts weigh in on that.
posted by mmascolino at 10:09 AM on May 7, 2007

Can't answer all your questions. Growing up in Northern Wisconsin though it was always recommended that one use a lighter oil in the winter. Not sure if that is still recommendation or if it's gone the way of snow tires.
posted by DieHipsterDie at 10:09 AM on May 7, 2007

Great site here. Everything you could want to know about oil.
posted by white_devil at 10:18 AM on May 7, 2007

Best answer: The viscosity matters, because that's what the oil is there for, its viscosity. Effectively, the oil creates a thin film, or boundary, between the parts of your engine that rub together. Higher number = thicker, which considering nothing else, is better. Thicker also means it takes higher pressure (harder on pump) to get the oil into tiny spaces.

Viscosity is a measureable quantity. In the old days, a 40 weight oil had a particular viscosity at 5 degrees, another viscosity at 50 degrees, at 100, etc. These numbers went down as the temperature went up. A 5 weight was much thinner (lower viscosity) than the 40 weight at any particular temperature.

Modern multi-weight oils have a more stable absolute viscosity. When they're cold (0C), a 5W30 has the same viscosity as a 5 weight oil at the same temperature. When the multi-weight oil is at 100C, 5W30 has the same viscosity as 30 weight at the same temperature. These absolute viscosities are very similar. Which is the long way of saying, the multi-"weight" oil maintains viscosity despite temperature changes.

Practically, what all this means is, most modern cas with tighter tolerances require 5W something just to make sure the oil's in the bearin spaces on startup. At operating temp, 30 is usually called for over 40 so there's less drag due to viscosity->slightly higher gas mileage. However, there's less margin of safety in extreme temperatures.

Mexican desert - go to 5W40 or even, 10W40.
Michigan - 5W30 or, if it's on the list of allowables from the manufacturer, 5W25
posted by notsnot at 10:25 AM on May 7, 2007 [1 favorite]

You don't mention which engine you have in your Jetta. Generally, you should be just fine running either grades of oil year-round. If you do your own changes, make sure you get an oil that is VW502.00 (or better) spec approved. You can find the specs listed on the back of the bottle.
posted by Thorzdad at 11:55 AM on May 7, 2007

Notsnot is correct, it's all about safe operating temperatures for you vehicule. The critical temerpature is how viscous the oil is when you start the car. Too much, too cold and your starter may have to work too hard. Too little, too warm and you may not have enough lubrication.

As notsnot says, modern oils are much less variable with temperature than older ones. Synthetics are very, very clean compared to a distillate cut oil. They don't burn as easily and have no residues to precipitate and form crud in your oil pan. Synthetic oil keeps your car much cleaner which means fewer changes are needed.

An 5W30 may be too light for desert use. Your mechanic is probably recommending a 10W40 oil. I really don't know. I'm a northern boy. I do know that using 5W30 oil makes a huge difference in say, Edmonton in mid-February. Do you want to get home tonight, or find a boost for your dead battery?
posted by bonehead at 11:56 AM on May 7, 2007

notsnot nailed it. Only thing I would add that may not have been made clear is that when choosing which oil to use, you have to consider two extremes: starting up your car on a cold night (the coldest temperature you might get where you are), and the hottest your engine might get (again considering prevailing temperatures).

If you go with a heavier oil, your car may be very difficult to start when it's cold, putting extra strain on your starter. Yet if you go with too light an oil, your engine won't get the right lubrication, because as everything heats up, it has less viscosity.
posted by Brian James at 1:21 PM on May 7, 2007

The cold viscosity rating will matter if you need to start your car in very cold conditions. It will make it a little bit easier to start, but it probably doesn't matter that much. Edmonton cold, maybe, Michigan cold, probably not.

Unless your engine is prone to overheating, you don't need to worry too much about the warm viscosity rating. Your engine should (unless something is wrong with the radiator) warm up to operating temperature and stay there, winter or summer. It doesn't really matter what the outside temperature is; it only matters what temperature your engine is. I suspect, given that you drive a reasonably new car, that your engine never gets much warmer than operating temperature, so it doesn't matter.
posted by ssg at 2:35 PM on May 7, 2007

Response by poster: Thanks all, esp. Notsnot and those that follow. These are all good explanations (and logically consistent too!).
posted by mdion at 2:55 PM on May 7, 2007