CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Use of diesel oil in gas engines - Good idea or not?

Ok, here goes another oil related discussion… I have heard several opinions on using diesel oil in a gasoline engine. Some say this is a great idea for the engine internals while others say it isn’t the correct application. All agree that the higher levels of anti-wear additives might shorten the life of emissions related components over time like the cat, O2 sensor, and the like.

I have put some diesel oils in some of my air-cooled equipment as it doesn’t have emissions components to foul. I have used Rotella synthetic 10W30 and 5W40 in these engines and know to avoid oil that might be way too thick. I figure these are harder working engines that operate in dirtier and hotter conditions that more approximate a diesel engine. I know others who do the same and no one ever complains about an engine related failure. I suspect that a big part of the life of these engines is actually changing the oil as I know many who just run them until they lock up or blow a rod through the side.

Are there any other main drawbacks to using diesel oils in a gas engine or not? I know others that run them in vehicles and everything seems fine for the moment. The only other thing I can think of is that the additive package may break down in the presence of gasoline combustion products as these are likely far different.

While not the best for gas engines, Diesel intended lubricating oil is just fine. older diesel fuels used to have a lot more sulfur and the oils for the engines had a high TBN or ability to neutralize the acids formed. THey also tended to have a more robust detergent package to handle the soot . Newer engines and ULS fuel means that the oils don’t need those additives and they are coming back in line with the gas engine purposed oils.
My best recommendation would be to by a good quality oil, I like synthetic but they are more expensive and change it at the reccomended intervals.
There are many good sites on the internet for motor oil info, bob the oil guy or something like that has several test of engine oils so you can choose the right price point to benefits for yourself.

EXXON has what they called a “mixed fleet” oil, which is OK for both diesels and gasoline engines.'s intended for businesses that have both types of vehicles and reduces their inventory they have to stock. Your industrial lubricants distributor would have it; don’t count on Walmart to stock it.

I understand that the Rotella T6 is a “mixed fleet” oil. It is a full synthetic and can be bought at Wal-Mart for $20-22 usually. I found a .PDF specs sheet that I would post the link to but it seems some type of filter on the forum doesn’t link links and my messages will not post. It meets the gasoline SH, SM, and SL specs.

cwatkin,thats good to know,most spark ignition oils now don’t even carry a c rating-Kevin

Yeah, this oil carries a ton of the C-ratings and I think the gas engine ratings are somewhat of an afterthought. Either way, it looks like you can use it if the viscosity ratings match the specs called for by your engine. I don’t think I have anything to worry about in simple air-cooled engines as I think the extra wear protection additives will help a lot.

@cwatkin - current (2010 and newer) is SN. What year is your car?
More info here:
http://www.api.org/certification-programs/engine-oil-diesel-exhaust-fluid/~/media/Files/Certification/Engine-Oil-Diesel/Publications/MOM_GUIDE_ENGLISH_2013.pdf

I’d not pay anything additional to use a diesel spec oil (that is also SN rated) than a correct gas-spec oil. Since you’ll never have a problem using a gas spec oil (assuming you follow the correct oil change interval), why bother with the diesel oil?

I know that some small engine owners have used diesel motor oils in their small motors with good claimed success. At least the motrs aren’t self distructing. Personally, I still wonder why we keep trying to reinvent the wheel. When manuals for gas motrs start recommend doing it, I’ll use them. Until then, the question needs asking AGAIN; why bother ?

The secret to a small air cooled motor in a lawn mower, tiller, or snow thrower lasting isn’t the oil, it is changing the oil and using the recommended weight. Diesel oil is fine, but so is basic 30W oil in most cases (snow throwers often take a 10W-30). Homeowners should change the oil once a year and rarely do it. Commercial lawn services likely know the importance of changing the oil and do it as needed based on hours.

Current diesel motors run much cleaner and using diesel rated oil in a gas powered car is fine as long as the weight of the oil meets the mfg’r specs and the oil has the proper ASI rating. If it costs extra money over regular oil I don’t see any real advantage. Again it is the oil change frequency that is the more critical factor. If you use diesel oil in an attempt to stretch your change frequency that strategy could backfire on you.

All the vehicles in question are year 2000 or older. This says something about the maintenance they have been getting as all run great and don’t use a drop of oil between changes. I have actually heard that some of the newer oils are not as good for older engines because of the reduction in zinc content. While this probably isn’t an issue in most engines from the past 20-30 years, it appears to be with older classic cars, especially ones with flat tappets. There is even a special oil with higher levels of zddp sold just for vintage autos.

As for the small engines, I agree that the main thing with them is simply changing the oil. Homeowners seem to like to buy the cheapest thing from a big box store and just run it until it croaks. This was my main potential use for the diesel spec oil as these engines worn hard in hotter and dirtier environments than most vehicle engines. Since they are air-cooled, they can get quite warm, especially during the summer when they are usually used. I wouldn’t use better oils as an excuse to skimp on changes, especially knowing that most of these engines do not have an oil filter. I think my plan is to run diesel oils in these and regular gas oils in my vehicles.

I pretty much run synthetic is everything, even my 2-cycle mixes. It doesn’t matter if it is a lowly big box store mower or a vehicle. I don’t mind paying a slight premium for this additional quality of oil and I usually stock up when I see it on sale or a good price at Wal-Mart. I have found that the price difference is usually not more than a few dollars per change when I buy oil this way.

15-40 Universal Fleet Oil has been around for a LONG time…Today, you can buy “Diesel Oil” in viscosity grades suitable for todays gas engines. If you look closely at the label, most of these Diesel rated products also meet the API SH, SN, SL specification used for gasoline engines. So why do the oil companies package so many different kinds of oil?? The answer is Shelf Space. The more shelf-space you have, the more product you will sell…Manufacturers frequently PAY retailers for shelf-space in one form or another…

The reason to use diesel spec oil is the additive package with zinc (zddp). New engines have roller tappets and don’t need the zddp. Older engines, pre-2000 or so with sliding cam followers will experience excess wear with car oil. Diesel oil meets the older car specs, as @cwatkin posts, so it will work fine in engines needing at least that spec oil.

The ratings started at “A” and have progressed to SN. I once bought a quart of “A” rated motor oil with the RockyTop motor oil brand just to prove the brand existed… it smelled like rear end lube and looked dirty out of the bottle!

Why would anyone second guess the designers? What’s to gain? Why lubricate your car engine with someting other than what’s recommended?

Modern engines are manufactured with much better parts fitment than old engines, the oils performs as a hydraulic fluid to operate the variable valve systems, modern cars have systems like the cat converter that were not an issue long ago, and modern oils are “designed” fluids rather than just pumped and fracked.

IMHO it’s a bad idea. And using the wrong lubricants will ultimately catch up with you. With zero potential benefits to using diesel oil in your gas engine, I don’t understand why anyone would try it.

In the never-ending shelf-space wars, it won’t long before EVERY different engine will require it’s own special oil…Hundreds and hundreds of “different” types of oil…We are almost there already…