In the old days, okay 35 years ago, I use to add diesel to my gas tank about a gallon or so in the 20 gallon tank and it ran better because of it, even returned better gas mileage...My Question is...With todays cars (i.e.) my 1995 Lincoln Town Car with 150,000 miles on it) will it hurt the catalytic converters or do any unwanted damage to my car? For some reason the car uses oil (about a quart every 5,000 miles. I just bought the car 7,000 miles ago and just switched to Synthetic Oil as the previous owner always used just conventional motor oil. I asked him about it and he said that the car has always done so and he doesn't believe all the hype about Synthetics. Please if you have the time respond directly to me at

Thank you,
Lindsey "Kansas" Angell

Older engines which were designed to run with traditional mineral oil will take no benefit whatsoever from synthetic oil, they are simply not built to process a substance that does not have the properties of regular oil. In fact, some older cars, due to more fragile seals, gaskets, and other vital engine parts, may have their engines further degraded by the harsher products that go into composing synthetic oil. In cases such as these, it is very possible and even likely that synthetic oil will damage your engine and drastically shorten the life of your vehicle.
Some old-timers and shade-tree mechanics will add small amounts of diesel fuel to the gasoline in their cars in the belief that it will lubricate the valves, in the same way that people will add such items as "Marvel Mystery Oil" and automatic transmission fluid to their gas tank. I can find no reputable studies that show any merit to adding diesel fuel to gasoline. In any case, the Auto manufacturer has certain recommendations for a reason. I would suggest following these only to inhibit your chances of finding a "suggestion" that could cost you more money in the long run.

Adding small amounts of diesel to your gas will not harm your catalytic converter and may even provide a slight increase in fuel mileage, since a gallon of diesel has more energy in it then gas. Depending on your driving style, of course. That's the up side. The down side is that you could end up damaging sensitive engine sensors, and you are altering your fuel's octane level, which can also result in possible damage or loss of mileage. Once again, depending on your driving style. You will also hurt cold weather starting, since diesel does not vaporize like gas in a cold engine.
The only down side to using synthetics in an engine that has been using a good quality conventional oil is something called "absorbtion rate". Lip seals (the type commonly used in modern engines) are designed to absorb some of the fluid they run in. This causes swelling which tightens the seal on the sealing surface. Over time they take a "set". Changing to an oil with a different absorbtion rate, such as synthetics, can cause the seal to shrink and pull away from the surface and leak. That's how these "seal restorer" additives work. They swell the seals. We ran into this same type of problem in the '80's when the switch was made from high to low sulpher diesel fuel. It affected the seals in the injector pumps of some diesels, notably Cummins. Changing the seal ends the problem, whether it's engine oil or fuel. Some engines will tend to use more oil when changing brands.I won't try to diagnose your oil consumption problem, other then to say that it is probably the result of prior maintenance, or lack there of.
Use good oil. Unless you are using oil sampling, change it at the recommended interval. Use good fuel. Try the different grades to see which works best with your driving style. Forget the "miracle" additives.

Carbon in the oil might be a problem. That is what ruined GM diesels as the were not adapted enough to handle it. I've only driven diesel for the last 29 years and the oil turns black with carbon very soon after a change. A good diesel engine has harder wear surfaces to handle the carbon. Also, I use oil for diesel engines which I think helps with the carbon. I would think your octane rating would go up.

I don't know any reason why you should use diesel in a gasoline engine.

But I know several things that can be harmful.

Let's first consider, as has been said here already, that the diesel fuel does not vaporise as easily as gasoline does. This and the fact that diesel doesn't ignite as easily as gasoline does have the consequence that it makes it more difficult for the engine to start and to run and to run cleanly.

So you will not only reduce the power of the engine but also increase the harmful emissions and both are undesirable effects.

The difficulty to vaporise the diesel fuel has also the consequence that the diesel fuel might not burn in the combustion chamber at all, but it can go down, past the pistons, and mix with the lubrication oil, which is at the bottom of the engine. This will result in the oil becoming leaner and less lubricative, which is obviously not a good thing. Small part of the gasoline also, perfectly normally, ends up mixed with the lubrication oil, but it isn't so serious as it vaporises more easier than diesel so it can vaporise away from the fuel for some part - and the amount of it in the oil in the first place would be less as most of it is going to be properly vaporised when entering the combustion chamber and would burn normally, causing no harm to the lubrication of the engine.

Neste Oil, a Finnish oil company ( http://www.nesteoil.com/ ), gives guidance that if diesel fuel has been used in a gasoline engine then the fuel and the engine oil and the engine oil filter need to be changed. They say that already a few % of diesel in the gasoline will cause the behaviour I describe above, that the fuel doesn't burn but it mixes with the engine oil. I take it from that advice that if you have only something like 1 % of diesel in the gasoline then most of it would burn (although causing the harmful emissions coming out of the engine to increase) and not harm the engine's lubrication (much).

Neste Oil also says that it will reduce the octane number of the fuel if greater amounts are added. If the octane number is reduced enough then there can be damaging effect to the engine because the fuel would not stand pressure as much as it should and cause premature ignition which is definitely harmful and can cause mechanical parts of the engine to break.

Here we are not yet talking of the damage that could happen to the catalytic converter. Catalytic converters are sensitive pieces of technology: if you have abnormal emissions coming from the engine, like if a lot of unburned fuel (gasoline is harmful also) is coming to the catalytic converter, then there will be increased burning in the catalytic converter and this could destroy it very quickly. If the catalytic converter doesn't work then it will result in that your car will fail an emissions test.

A summary. By using diesel in a gasoline engine, you will, at least, but probably not limited to, cause these harmful effects:

A) Increase the wearing rate of the engine.
B) Reduce the engine power.
C) Increase the harmful emissions coming out of the engine.
D) Make it more difficult for the engine to start.
E) Make it more difficult for the engine to run.

I cannot think of anything positive why you should use diesel in a gasoline engine.

I DO NOT RECOMMEND USING DIESEL IN A GASOLINE ENGINE! DON'T DO IT!


All 7 years of my formal education past elementary school is from the field of automobile technology. Just trying to assure you that you should take this advice, it comes from someone who has studied the designing of internal combustion engines etc. subjects.