Can I mix different weight oils in my car?

April 17, 2006 8:40 AM

I just got the oil changed in the car. I usually get them to put in 20-50, but this time they only had 15-45 or whatever it is. Problem is that I have about 10 quarts of the former and none of the latter in my trunk. When I top off the oil in the coming months, can I use the heavier stuff, or should I get the lighter?
posted by zorro astor to travel & transportation (11 comments total)

You can mix different grades of oil with no bad effects. Do not, however, mix synthetic and normal oils (unless the synthetic's manufacturer specifically says it's OK).
posted by Kirth Gerson at 8:52 AM on April 17, 2006

Whoa. 20-50? I've never used anything but 10-30 or -40. Where exactly do you live? Somewhere extremely hot? Even so, 20-50 is bad on almost any car. Maybe you have some kind of diesel rig, I don't know. Look here or here.

I've never heard of mixing weights causing a problem... unless, of course, neither of the weights being mixed is good for the engine.
posted by Viomeda at 8:53 AM on April 17, 2006

On reflection, the probably only had 10w40. That's a really common oil weight. My suggestion: throw out the 20-50, and stick with 10-40 or even 10-30 in the future.

As far as topping off, most oil-change places worth their salt will do it for you, especially if you got your change there.
posted by Viomeda at 9:05 AM on April 17, 2006

In general, newer engines run on much thinner oils. Most new cars today run 5W30 and most oil change shops use 10W30 for everything. (the first number is cold viscosity and the second number is hot viscosity -- bigger numbers are thicker). 20W50 is extremely thick and would be unsafe in a newer car or a rebuilt engine because if the bearing clearances are tight, this might increase oil pressure to unsafe levels.

In an older car, especially one that uses oil, people often go to thicker oils to limit oil consumption and because it minimizes the risk of overheating and blowing up. If you have an older car, I'm not opposed to running oil this thick, especially if it is one with known problems with engine oiling. You almost certainly will have plenty of bearing slop in an older car.

A newer car should almost certainly not run oil this thick. If you want to effectively thicken your oil over time, there is no problem with mixing a thicker oil with the thinner one that is already in there.

Some people believe that you should never mix synthetic and petroleum oils, but I actually doubt that there is any scientific basis for this.
posted by Lame_username at 9:45 AM on April 17, 2006

I assume zorro carries oil in the trunk of the zorromobile for the express purpose of topping off what's in the engine. Why, then, would zorro go to an oil-change place and pay them to do something he or she is fully capable of doing? I further assume that zorro had good and sufficient reason for having 20W50 oil in the first place.

Adding one quart of heavy oil to three or four quarts of lighter oil is not going to have any bad effect. Throwing away perfectly good oil, on the other hand, is going to have a definite bad effect on zorro's finances.
posted by Kirth Gerson at 9:53 AM on April 17, 2006

My comment re: mixing synthetic and mineral oils may have been out-of-date. However, from here

Are there any drawbacks of mixing mineral and synthetic fluids?
Answer: Yes! Reduced service life for the mix, and more harmful oil breakdown products- the types which cause more sludging and corrosion. Mineral oil has less oxidation resistance than synthetic oil. The mineral oil will oxidize faster, and produce oxidation byproducts. These byproducts act as catalysts towards further oxidation, which will induce the synthetic product to start the oxidation degradation also. The result is a more shortened life than what you would expect for the full synthetic base. The byproducts formed when the synthetic oil breaks down are generally more corrosive than the mineral product.

The easy fix here is don’t mix the products. When ever you are switching from mineral base oil to synthetic base oil, be sure to thoroughly flush the old product out to ensure maximum service life for your new synthetic.

posted by Kirth Gerson at 10:04 AM on April 17, 2006

Lame_username: "Some people believe that you should never mix synthetic and petroleum oils, but I actually doubt that there is any scientific basis for this."

The "scientific basis" is this: the benefits of synthetic are all nullified when it's mixed with natural oil, and synthetic costs more. Therefore, if you have a car that's always run on synthetic, you'll prefer to keep it that way, whereas if you have a car that's never run on synthetic, you'd rather not waste the money.

Kirth Gerson: "I assume zorro carries oil in the trunk of the zorromobile for the express purpose of topping off what's in the engine. Why, then, would zorro go to an oil-change place and pay them to do something he or she is fully capable of doing? I further assume that zorro had good and sufficient reason for having 20W50 oil in the first place."

Because most oil change places do it for free. As far as having sufficient reason for using 20W50, sure, it's possible. Sorry to zorro for the derail, if the suggestions are unwarranted.
posted by koeselitz at 10:08 AM on April 17, 2006

Mixing grades is not the problem, neither is synthetics and crudes (though it is true that you will lose the expensive benefits). The real problem is in mixing detergent and non-detergent oils. Never mix an oil that is rated, say 10-30 or 20-50 with one that is rated, say, SAE30 or SAE20.
posted by Pollomacho at 10:27 AM on April 17, 2006

the benefits of synthetic are all nullified when it's mixed with natural oil

Which is why pretty much every oil company sells a natural/synthetic blend, I guess.
posted by kindall at 5:55 PM on April 17, 2006

My car is a 1987 Toyota pickup, 300,000 miles + on an engine never rebuilt. I've been told to use the thickest stuff possible.
posted by zorro astor at 6:22 PM on April 17, 2006

kindall writes "Which is why pretty much every oil company sells a natural/synthetic blend,"

Marketing hype, I'd bet that the percentage of synthetic in the oil company blends means you could mix your own for a lot cheaper. Also it allowed them to introduce a "new" product to WalMart's buyers which means a product that hasn't been roll backed to the point of no profit.
posted by Mitheral at 8:41 AM on April 20, 2006