Grading Motor Oils

courtesy of Bob, The Auto Answer Man


Hi Bob,
What is meant when oil is labeled 10w40 or 10w30 or 5w40? I live in hot New Mexico where summer highs peg 115 and winter lows get to the "burr" 30s. What is the best type of oil to go into my 95 Corsica?
David L

Dave,
I think the WHOLE world is in trouble! Why? 'cause people are actually listening to me! I don't claim to be a guru (or a mechanic for that matter). I just happen to be a guy who likes cars and knows a few things! I actually hold a degree in Electrical Engineering Technology. I am currently the Lab Manager of a Automotive emissions research firm located in New Jersey. Any advice or explanations given are to be used only as such and should be evaluated by your own experiences and knowledge. If anything seems unclear, then refer to you local mechanic. Remember, that I am only "diagnosing" via a written description of a "problem" without actually seeing the vehicle or knowing its history. For exact procedures on how to do anything for your vehicle, I refer you to your vehicle's owners manual.

Okay, now that we have all that legal mumbo jumbo out of the way, we can get back to your question. How to decode the numbers on an oil can. Well, to put it into laymen's terms... a 10w30 oil is an oil that flows like a 10 weight oil but lubricates like a 30 weight. So a 10w30 and a 20w30 and a 30 all lubricate the same, but the 10 flows easier. Okay, now what is best for your climate. First thing to do is go to your owners manual. Look in the index for lubrication. On that page you will find the recommend oil viscosity rating. Generally for average climates, a 10w30 or a 10w40 will suit most cases... for a colder climate a 5w30 is necessary... for your desert climate I would imagine that the manufacturer would recommend a 10w40 oil. In newer cars, I do not believe that a 20w40 or a 20w50 is necessary. You might, however, consider installing an oil cooler. This device acts like a radiator but for your oil. It also looks like a radiator for that matter. Also, change the oil frequently. At higher temperatures, the oil breaks down faster. Also if you happen to live in the desert or anywhere it is extremely dusty, again change it more frequently.

Another reader asked me about synth oil and whether or not they are good and if they can be mixed with conventional oils. Well my answer to that is this. First, I am not a lubrication engineer, but as far as my knowledge goes, only some synth oils can be mixed with conventional oils. I know valvoline makes a blended oil as well as castrol. Mobil makes a synth that we all know, MOBIL 1. This can NOT be mixed with regular oils. Castrol makes a new oil called SYNTEC. It comes both in blended and 100% synth. I like the 100% version. According to the literature, it can be mixed with regular oils and it has "Specially formulated molecules that cling to the metal surfaces". In English, I really don't know exactly what that means, but there is some sticky stuff that hangs on to the metal better. It is unlike Teflon in that is actually part of the oil and not some semi solid mixed in. This oil is supposed to give superb cold start performance. Incidentally, this is where most of the engine wear occurs.

In closing, I hope you are learning the "right" stuff. I never claimed to be a teacher or philosopher (Plato I'm NOT). And on occasion happen to be completely off the wall. But what the heck. As long as no one gets hurt and we all have fun. I understand that a lot of us out there need to watch every penny. I try as best I can to help all of my readers. Some letters that I receive, there is absolutely no way that I can even begin to figure out what the cause or problem could be. Sometimes it could be one of a hundred different things. So if I don't answer your letter, don't send it twice. I just might not know!
Good Luck!
Drive Safely!
Bob, The Auto Answer Man


If you have a question for Bob send it to: . He's able to answer many of them personally and we'll include the best questions in future issues of The Dollar Stretcher.

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