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Old 04-18-2004, 11:39 PM
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Demystifying Oil - General high-level info about oil

Please note: This not a recommendation, suggestion or even inference of what is better or worse, simply some basic oil information


Demystifying Oil - What do all those numbers and letters mean, and are all brands the same?

Are you puzzled about the types of motor oil available? Wondering if you're choosing the right one for your car? Here's a crash course, so to speak:

Multi-viscosity or straight-weight. Multi-viscosity oils are those denoted by two numbers with a 'W-' in between (for example, 10W-30 or 20W-50). The 'W' originally stood for winter, as oils were developed to compensate for the wear due to the vast temperature ranges then. They're almost exclusively used today rather than straight-weight (monograde) oils. Multi-viscosity oils use polymer additives to change the natural consistency of oil as it's heated, preventing it from thinning out as much as it otherwise would.

The only downside to this is that some of the additives used to improve viscosity at high temperatures can also leave sticky deposits. Oil companies have mostly eliminated such problems by adding detergents to oil, along with rust inhibitors and antioxidants. Some precise performance engines will still require straight-weight oil, but all newer vehicles are designed for multi-viscosity oil and require it, else your warranty will probably be void.

What the numbers mean. The numbers in motor-oil nomenclature refer to the viscosity (thickness) of the oil. A higher number corresponds to thicker oil, while a lower number refers to thinner oil. The numbers themselves are supposed to correspond to a set of real, measurable qualities in the oil, one of which is the viscosity index. In multi-viscosity oils, the left number refers to cold behavior of the particular oil, while the right number refers to its hot (100 degrees Celsius) behavior. So, for instance, 5W-30 oil would flow well when cold like 5-weight oil, but protect at high temperatures like 30-weight oil.

Do oils vary much by brand? Yes. The 5W-30 oil from one company might equal 10W-40 oil from another company in viscosity, because there might be a difference in some of the other properties of the oil, like its flash point (at what temperature it ignites). The numbers in multi-viscosity oil also don't tell anything about how viscous the oil is at normal engine operating temperatures, as opposed to extremes. The best advice here is to choose a familiar brand, and experiment with other major brands if you think it's too thin or too thick. Never mix brands or different weights. Wait until an oil change.

What about synthetics? Synthetics are better in every way, but they are much more expensive. They offer better high-temperature resistance and better low-temperature flow, and they leave nearly no deposits. Prices for synthetics are coming down, though, and there are also blends that combine synthetics with traditional mineral oils. Because synthetics flow and penetrate much better than regular oils, a change to synthetic oil will sometimes reveal leaks you didn't know existed.

Ratings. The American Petroleum Institute (API) assigns letter ratings to motor oil. The category ratings correspond to industry standards for various qualities like viscosity, thermal protection, and preventing deposits and sludge. Every few years, the currently awarded rating is changed. Currently, SL (Spark ignition, Category L) is the top rating. SJ was introduced in 2001 and SH in 1996.

As your car ages and becomes a high-mileage vehicle, many experts recommend that you change to thicker oil than is normally used. Over time, gaps between parts in the engine become larger, enabling less oil to reach critical parts. Ask a mechanic familiar with your model vehicle or ask the dealership if you should change the type of oil for your vehicle beyond 100,000 miles.

The underlying message is that all oil is not the same. Making sure you use the correct viscosity rating is just the start. The quality and characteristics of oils can vary greatly by brand, too.


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Old 04-19-2004, 09:11 AM
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Thanks for the good information.

The only thing that I would like to point out is that using a heavier oil at a given mileage could be a very bad thing. Granted, once an engine gets worn significantly using a slightly heavier can, in some cases, extend engine life a little. But that is once it is worn. Engines wear at extremely varying rates.

An engine that goes one mile to the office and then one mile home every day will wear at an extremely high rate per mile as compared to an engine driven exclusively on long trips will wear very slowly per mile.

The sign that it is time to use a different oil weight should be the regular wear signs, not mileage. These signs are oil usage, compression and oil pressure.

My 300E for instance is at 267,000 miles and still holds strong oil pressure, uses absolutely no oil and has very good compression. Going to a heavier weight oil would actually accelerate wear during cold starts.

My $0.02,
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Old 04-19-2004, 12:35 PM
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I wouldn't advise changing oil viscosity based on mileage either. Americans still insist on paying a premium for low mileage cars, when in fact the engine is probably shot from putzing around town. Short distance driving is considered severe operating conditions. If you don't believe me, look in the little book that came in the car's glove box. Diesels suffer the worst, because of the high compression. I bought my 83 wagon dirt cheap with 225K on the clock. I just changed the oil at 335K and use the same 20/50 I've always used. Oil pressure normal, fires right up on cold mournings, and no oil usage except for those pesky leaks. I've done the cooler lines and filter housing gasket. I just can't see pulling the pump to change that gasket.

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Old 04-19-2004, 01:34 PM
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Cold mornings in Northern Calif. ?
I haven't laughed so hard in a long time.
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Old 04-20-2004, 11:11 AM
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Well here in the Bay Area it does get way down to about 34* F a few mornings a year. Don't you think that is pretty cold?

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Old 04-20-2004, 12:20 PM
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AutoZen: How about MINUS 34 degrees???
In our neck of the woods, at your 34 degrees, it is time for us to clean the pool and have a dip.
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Old 04-21-2004, 03:28 AM
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here in chicago/nwindiana we're not quit that tough but when the temp gets down below 32 we figure it's about time to go in and put a shirt on and maybe some long pants.... if it's night time.

and autozen i was born in calif and any state that can bloom a rose in february or at christmas be 85 degrees with santa ana winds hasn't seen cold yet.

only funning but it was good for a chuckle

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