All Motor Oils Alike?

I ask because I’ve heard two opposing opinions:

  1. All motor oils (which carry the SAE certification stamp) meet the standards, and are equally as good
  2. certain brands are better. One in particular (CASTROL) seems to be favored by foreign car dealerships. Castrol is a relatively small oil produced-so is its product really better than Mbil ot shell’s oils?
    My mechanic says that any branded motor oil is OK, as long as you change it according to your car’s instruction manual. Who is right?

Stop! You’re both right!

Motor oils which meet the appropriate standards are adequate. In that sense they’re “equally as good.”

There are some differences among brands. For example, field experience has shown that engines using brand A tend to get more throttle body deposits than engines using brand B.

As far as “better,” define better. No doubt some brands are better than others in certain aspects. As far as engines lasting longer with brand C than with brand D, all other things being equal, I question whether there’s solid proof. Maybe someone will link us to said proof, maybe it doesn’t exist.

There’s a lot of superstition and tradition behind people’s perception of motor oils, and a fair amount of misinformation related to that.

Bottom line, though, your mechanic is right in that performing proper regular maintenance is the key. So long as you use a quality oil, the particular brand almost certainly doesn’t matter.

With ten years and too-many-too-remember classes on just this kind of thing, I’m of the belief that all petroleum oil products are the same. You pay for the marketing.

Synthetic, that’s a different beast, altogether.

Oh, I worked at Grand Auto. :smack:

Changing your oil & oil filter on a regular basis is 10,000,000 times more important than the brand of motor oil to use.

A friend who works in auto mechanics told me that Castrol is indeed better, from the dealerships viewpoint – it sells to dealers at a lower price, so provides them with a higher markup and bigger profit. They consider that “better”.

I’ve spent several hundred hours studying this topic.
I’m compiling a database, currently at 400 entries, of all the world’s publicly available chemical used oil analyses.
Gary T is spot on.

If you avoid non-certified oils not intended for your application you’ll have substantial trouble telling the difference between one appropriate oil or another based on vehicle wear, fuel economy or vehicle performance.
If you plan on running your oil for 15,000 miles, then there are some differences between brands, likewise for starting at temperatures much colder than you get in Chicago in the winter.

Regular oil changes on my old truck took it to 395K miles. Pennzoil, Quaker State, Castrol-whatever 20W-50 was on sale was bought by the caselot, along with Fram or Purolator filters. It’s hard to screw up with regular oil changing and chassis lubes.

Which brings up the related question of viscosity. It seems that a lot of modern vehicles call for lower-viscosity oil, but a lot of do-it-yourselfers seem to prefer something thicker. What’s the straight dope there?

Input is welcomed-I chose to run higher viscosity product because my vehicle spends 75% of its mileage on limited access roadways running at 65 MPH. If smarter guys weigh in and advise otherwise, we’ll all learn.

I seem to remember that Consumer reports did some tests on this and found that all the oils natual or synthetic worked equally well.

Are all brands enough “the same” that mixing brands (but not weights) of oil is OK?

Yes, mixing brands is okay. It has probably always been okay.

Mixing viscosities (“weights”) is okay, in that the mixing itself doesn’t cause any harm. It makes it hard to evaluate what the resultant viscosity rating might be, though.

When detergent motor oils were first introduced as the new and better thing, they were often used in cars that had sludge build-up from the non-detergent oils they’d been run on for years. Sometimes the detergents dislodged the deposits which then went on to block critical internal engine passages, with understandably disastrous results. From this came the advice “Don’t mix types of oils.”

This was good advice when the types were detergent vs. non-detergent, in the context of not putting the former in an engine that had long run on the latter. Unfortunately, this got misconstrued to mean “Don’t mix brands” and “Don’t mix viscosities,” neither of which has ever been a problem to my knowledge. This is a classic example of the “superstition and tradition” I referred to above - no one can say why or how mixing brands/viscosities is a problem, but they’ve always heard not to do it. So the misinformation gets passed along by people who ardently believe it, even though there’s no evidence to support the notion.

For those of you that have the same engine type as those taxis had, and the same use patterns, you can probably expect similar results.

Do not, however, think that all engine designs are as insensitive to the viscosity used, nor that all use patterns put oil to the same test. For example, consistently use 20W-50 in a vehicle that experiences a lot of cold-engine starts (how often are taxis shut off?), and I doubt you’ll be so lucky.

Vary from the approved/recommended viscosities at your peril.

From years back, I remember a chemical that allowed you to run your engine without oil. Not that they wanted you to use it without oil but they had commercials that showed the engine ran for so many hours without oil. They even sold it in our local automotive store. What’s up with this stuff? Do they still sell it?
Sorry for the hijack.

It’s just an advertising gimmick. You add the miracle product to the oil, run it awhile, then drain the oil out. The engine will then run for a time without oil. But - you can do the same thing without adding the miracle product. Remember, it’s not operating the car in normal use. It’s a carefully constructed demonstration designed to make it appear that the miracle product is wonderful. Deceptive horsecrap.

Right again.

Today’s automotive engines are engineered to much tighter tolerances than those of previous generations of cars, and it’s more important than ever to use the correct oil. Piston rings , for example, are designed with a specific oil viscosity in mind, and variation from that specification can result in much unhappiness.

Larry, is that you?

You could’ve put margarine in the crankcase of that ol’ tractor motor and got it to 400K. Today’s engines are a orders of magnitude more demanding, and that 20W-50 will likely give your new car a severe case of indigestion.

Right again.

Today’s automotive engines are engineered to much tighter tolerances than those of previous generations of cars, and it’s more important than ever to use the correct oil. Piston rings , for example, are designed with a specific oil viscosity in mind, and variation from that specification can result in much unhappiness.

I don’t work for a motor oil company, but I do work for a company that makes the additives that motor oil companies put into their oil (specifically, I make the detergent).

Not all motor oils have the same certifications. In the US, the SAE sunburst is the most common one you’ll see, but there are other certifications. In particular, some European automakers have their own, preferred certifications. If you’ve got an unusual car, you’ll want to look for the certification your carmaker recommends.

That said, two motor oils from different companies but with identical certifications are essentially interchangable. The specific chemicals that’re being added to do the job may be slightly different, but there’s not enough profit margin to add any more than is strictly required to pass the engine tests for certification.

Mind you, those engine tests are notoriously fickle, and everyone in the industry argues if they really predict actual performance. But they do ensure that most motor oils are very similar.

What you want to look out for are premium motor oils that make un-certified claims of performance. In particular, motor oils for “longer-life” cars. These do have extra additives in them, but not enough extra to justify the premium that you’re paying.

Beyond that, I use synthetic motor oil, but that’s more of a personal prejudice against the naturals than insider knowledge.

As for change intervals, you basically want to change your motor oil when its basicity is used up (in neutralizing the acids generated by oil degradation). The overbased detergent is usually the component that’s going to be used up first. Unfortunately, unless you have a fleet of expensive trucks, its just not cost-effective to keep testing your oil – its easier to change it at the recommended mileage, which is determined by average driving, with a generous margin on the “change early” side.