Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

10/40 or 10/30

I have a car with 118,000 miles and everytime I go to have my OIL change they put 10/40w in my car. I was always told thay 10/40W provides limited protection. Is that true?

The answer is in your owner’s manual for sure, and it may be on the oil filler cap.

That being said, very few cars call for 10W-40 these days. What kind of car is this?

and still ANOTHER post without basic basic BASIC information.

If you have an older car that’s starting to burn a bit of oil, 10-40 in the summertime won’t hurt anything…In smaller, high-reving engines, it can reduce fuel mileage a little because of viscosity drag. In larger engines, this is not noticeable…

What is the year, make, and model of your car?

The answer is in your owner’s manual. Use whatever is listed there. The required oil is also listed on your oil filler cap. Putting it on the oil filler cap makes this decision a no-brainer.

10/30 and 10/40 both start out at 10 weight oil. As the oil heats up long chain polymer molecules heat up and they become uncurled. This makes the oil thicker. The thicker the oil has to be the more long chain polymers. The more long chain polymers a oil has the LESS oil it actually has. These long chain polymers are NOT very good lubricants. Some manufacturers have banned 10/40 from being used. The rule of thumb is to not use a multi-weight oil with a range greater then 20 (i.e 10w-30). Now that was about 5 years ago…Oils have improved since then. So not sure what the standard is now.

I used 10W-40 in my 1978 Oldsmobile Cutlass 4-4-2 which I bought new and still have. The owner’s manual said that I could use either 10W-30 or 10W 40. However, I had quite a problem with carbon build-up in the engine. I had to frequently pour Casite Motor Tune-up through the carburetor and add another can to the gas tank and then really do a hard run on the Interstate to remove the carbon. On CarTalk, Click and Clack told a caller that had the same problem that the 10W-40 might be causing the problem. It seems that with certain brands of 10W-40 the polymers would cause the carbon build-up. After hearing the call, I switched to 10W-30 and the problem cleared up. This was back when the car had about 50,000 miles on the odometer. It now has 240,000 miles and since the 50,000 mile mark, I haven’t had to run Casite through the carburetor. I would stay away from the 10W 40.

10W40 will provide more protection in hot weather than 10W30. It will be the same in summer. In the old days the additives used to make 10W40 were a bit of a problem and most of the oils failed to provide the same protection as 10W30 and they tended to loose their “40” specification as the aged in the engine. Modern oils don’t have the same problems. Don’t worry. Use what is specified in the owner’s manual!