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Dumb question:How do I switch from 5w-30 for 10w-40?

Like the title says, this is a dumb question but I’m new to working on my car so I hope someone can give me some advice.

I have a PT Cruiser that I’m in charge of maintaining. It uses 5W-30. There was a sale on 10W-40 so I bought some. I understand you are not supposed to mix oil grades. Here is the question, when you drain your old oil there is always some oil left in the pan, right? How much is too much leftover oil before I add the new grade of oil?

So, I guess the question is, should I do anything special before adding the new oil?


It is perfectly all right to mix grades of oil. If someone told you otherwise, you were misinformed. So drain the old oil, close the drain and immediately refill with fresh oil.

You can mix or switch to your hearts content without any worry.

No just change it. Now beware you just open a can of worms on this forum. You will have people telling you 5-30 is what your car calls for or only do what the manual calls for. If you want to use 10-40 it will not hurt anything. The choice is yours. Oh and you can mix oil grades

Simply draining the oil sump, changing the filter will probably get at least 98% of the previous weight out, but if it’s not ready for an oil change and you needed to add some for topping it off, mixing different weights or even conventional and synthetic won’t hurt a thing.

Unless you live in Death Valley or South Florida, you are well advised NOT to put ALL that oil in your car. It needs 5W30 or something close to it. Read the manual.

You CAN mix oil grades, as long as the viscosity (weight) of the oils mixed is within the range recommended. So 2 quarts of W20 plus 2 quarts of W40 will result in a W30 weight.

Since you have already bought it, you can add 1 quart at the time when the oil needs topping up. That way you will get the equivalent of 2x30+40/3 or about 33.3 weight. Since summer is approaching that’s OK.

If you read the container carefully, you may find that this oil may or may not meet the API (American Petroleum Institute) spec for your car. However, 1 quart at the time is OK.

If you lived in Minnesota and used this oil in the winter to fill your crankcase, you would have severe starting problems and have greatly increased engine wear due to the thickness of the oil.

Similarly, when your mother bakes and the recipe calls for vegetable oil, she would not use lard, even if it was on sale.

Well, I’ll be… That’s great to know. I googled my question and a lot of sites, said mixing oil grades was one of of the worst things you could do.

@Docnick:Is it really that excessive? I live in Portland. Right now the weather is very nice and mild but later in the summer it can get pretty hot. This car does a round trip of 50-70 miles a day on freeway. Sometimes it gets stuck for quite a while in the freeway. That’s why I thought that switching it to a heavier oil grade would b e ok.

Just curious, what sites warned against mixing grades? That really is incorrect information (edit - assuming either grade is OK for the engine, of course).

Oh and the oil is API certified. It’s Mobil brand.

The use of 10W40 motor oil was phased out by vehicle manufactures during the 1980’s when piston ring quality was improved to pass the EPA 100,000 mile emission durability test. 10W40 has a high amount of viscosity index improvers that may leave deposits in the piston ring groves.

The following statement is from a March 1984 GM bulletin;

SAE 10W-40 is not included in the viscosity chart for 1984 GM gasoline engines, for several reasons, including:

^ Fuel economy with SAE 10W-30 oils is about 1% better than that with SAE 10W-40 oils.

^ SAE 10W-30 oils generally produce less piston and ring deposits than SAE 10W-40 oils. Such deposits can lead to stuck rings and increased oil consumption.

The use of SAE 10W-40 engine oil in a vehicle equipped with a gasoline engine will not automatically void the General Motors Corporation New Car Limited Warranty. However, since SAE 10W-40 engine oil is not recommended for use in 1984 GM
vehicles equipped with gasoline engines, if the use of such oil causes damage, the cost of the engine repair will not be covered by the Warranty. The Warranty does not cover any damage due to the use of non-recommended oils, whether SAE 10W-40 or some other viscosity. In addition, the Warranty does not cover damages caused by the use of oils that do not meet the recommended API quality or SAE viscosity grade.

@texases:I found mostly forums with google. That’s why I wanted more info.
I guess that settles it,Nevada_545.

It seems like using this oil grade in this PT Cruiser is a bad idea. Do you think it would be ok to use it on an older car? Maybe I should just go back an return it…

Nevada_545 is right. I had a 1978 Oldsmobile Cutlass that I purchased new and sold this past October. The owner’s manual allowed the use of 10W-40, so I used that. I did have a problem with carbon deposits on the piston rings. About once a month, I would have to run some Casite Motor Tune-up through the carburetor and add another can to the gasoline. I would then take it out and run the engine in a lower gear at higher rpm to blow out the carbon deposits. After the bulletin came out, I switched to 10W-30 and did not experience this condition again. The car never used oil in the 240,000 miles and 33 years that I owned it.
If I were you, I would not make it a practice to use the10W-40 in your PT Cruiser. I would not even put it in at all, but if you do, use it up this summer and then go back to 5W-30.

Using 10W40 for one oil change won’t do any harm, it will take many years. You could also use it to top off the oil level between oil change, one quart won’t make a difference.

I guess it would be safer, but I myself wouldn’t worry too much. I remember when 10w-40 came out, and a number of tests pointed to those kind of problems, but I believe more recent tests (post 2000) have not shown any of those problems.

I remember when a caller to CarTalk mentioned having a problem with pre-igition (engine ping) over 30 years ago on a GM car. Click and Clack asked if the caller was using 10W-40 oil in the car. The caller answered that he was. Click and Clack referenced the bulletin. According to them, certain brands of 10W-40 oil had polymers that would cause carbon build-up, so GM issued the warning and recommended not using any 10W-40 oil. Texas is, no doubt, correct that this problem has been addressed and certainly 10W-40 oils have been improved over 30 years. I still wouldn’t put it in my car, even though the chances are slim about having a problem.
Whatever you do, don’t use it in your lawnmower or a small air cooled engine, unless the oil is synthetic. I have seen two rod bearing failures in these engines from using multiviscosity oil.

Mid 80s GM factory manuals indicated that above 60* ambient, 5W20 and 5W30 oils were discouraged. It’s such a ridiculous confusion of opinions that no one can be sure. But as for blending oils, I have seen vehicles that were serviced with the dregs from empty oil bottles and cans of all brands and weights. The engines seemed to be perfectly happy with the left overs.


In the summer a 10W40 just gets you a little less gas mileage. In cold weather, however, it takes a very long time for the oil to reach the valve gear on top of the engine. EXXON did a test with 15W40 and 10W40 at -35F and it took several minutes to get good lubrication to the valve gear. With a 0W30 synthetic, the oil flowed very quickly. The so-called “pour point” an oil is just that; the temperature where it will pour like maple syrup. A 10W40 at -35F will pour like molasses.

As others point out, the really thin oils used today are specidfied to get car makers to comply with fuel economy eequirements. My Toyota now specifies 5W20 or 0W20 synthetic. I normally put in 5W30 since that was originally specified for this engine.

Your engine is a relatively simple one, and any brand 5W30 on sale is good year round. Oregon does not get severe winter temperatures, so your 10W40 will not do a great deal of damage. Still, I would only use it in warm weather.

10w40 isn’t likely to create problems with your engine. I use 10w40 in all my cars year round most of which the manufacture recommends 5w30 and 1 that recommends 5w20. I also have about 20 cases on hand in my garage for future use. One of my cars has in excess of 500K miles, 2 near 200K, and two with between 100-150K miles with no signs the heavier oil has had any negative impact. I’ve also mixed various brands and weight in emergency situations and have changed brands multiple times in the past 24 years. I’ve read that 15w40 is the best weight oil for fuel mileage and engine longevity and is the weight most over the road truckers use.

I have a question. What parts store only puts one weight of oil, like 10W40, on sale? I look for sales all the time, and the ones I’ve found and take advantage of sell the BRAND and Subproducts, like synthetics or high-mileage, but these sales typically cover the spectrum of oil weights, like 0w-20 to 20w-50 or straight 30. I use these sales to stock up on oil, since buying it off-sale runs like $5.00 a quart. But, usually I can find an oil change special that includes 5 quarts and a filter for $15.00. And, I buy the weight that I’m low on, since the sale covers the spectrum of weights offered by that brand.