As the title says.
When buying engine oil there is a choice depending if your car has a diesel or petrol engine.
What is the difference and does it really matter?
Would there be any long term damage by using the wrong oil in your engine?
Just to satisfy my own curiosity so don’t need answer fast 
Additive packages. Diesel engines generate a lot more soot so the motor oil needs a higher amount of detergents to handle it. Plus, up until recently diesel oils could have higher levels of anti-wear additives like ZDDP, but since modern diesels have catalysts in the exhaust that’s been sharply reduced (phosphorous poisons catalysts.)
Ok I get that.
So putting “petrol oil” in a diesel engine would not do such a good job, what about “diesel oil” in a petrol engine would that be over kill but not harm the engine?
Engines manufacturers recommend service grades for the oil used n their engines. Deviating from that recommendation is a bad idea.
Some modern diesel car engines are manufactured to much closer tolerances than gasoline engines in order to operate efficiently, and this requires high grade oil with specific characteristics. I’ve heard that it also requires more frequent oil changes adding to the operating costs of these cars also, but I haven’t seen that verified. I’m not sure I understand it correctly though, these diesels are running at higher RPMs than comparable gasoline engines, and possible internal cylinder pressure is higher than you’d find in gasoline engines, but at the same time I understood the operating temperature should still be lower than gasoline engines. Maybe someone can verify this and explain the details. But even gasoline engines are running faster and built with closer tolerances than in the past and the specific oil used can certainly effect the efficiency of the engine even if not resulting in damage.
Diesels definitely run at higher pressures (they’d have to be as they function on compression ignition), but they don’t run at higher RPMs than gasoline engines of comparable output. They simply don’t have to run at high RPMs because they generate sufficient torque at low RPMs, the engine would be costly if they strengthened it for higher revs, and it would eat the fuel savings advantage for the consumer. Racing diesels have no such concerns so they go to higher levels, but they still get better fuel mileage than a comparable gasoline engine.
But to answer the OP, the oil run in the engine should be the recommended oil. A failure with a different oil probably wouldn’t be covered by warranty.
Generally diesels are not very high revving, a gasoline rated engine oil will have anti-foaming agents to prevent frothing in high speed use, those additives will not be present in a diesel engine type oil, and the detergent package will be different.
Generally speaking though, a diesel rated oil is at least “acceptable” in a gasoline engine or at least moreso than the other way around. Many people in fact run a diesel rated 15w/40 year round in everything they own, from gasoline tractors to weed wackers and everything in between, saves money and confusion. Delvac and Rotella is good stuff and in convenient 1 gallon containers.
Diesel engines have significantly different emissions control systems. Diesel engine oil has a higher anti-wear load in the form of particular type of Zinc (ZDDP). Zinc (as well as other toxic byproducts of the combustion cycle such as lead and phosphorous) can be damaging to catalytic converters. For this reason alone, you don’t want to use diesel engine oil in a gasoline engine.
Viscosity (thickness) is the single most important property of any lubricant. Diesel engine oils have a higher viscosity than oil for a gasoline engine. The ability to be pumped through the engine at lower temperatures is critical. Because it is thicker, diesel oil takes longer to circulate and will cause premature engine wear in a gasoline engine.
Diesel oils also have more additives, primarily to neutralize acids and clean the system. Diesel engines create a lot mroe combustion byproducts than gas and some of this can find its way to the crankcase, requiring the oil to deal with it. When the additive-laden diesel oil is used in a gasoline engine, a reduction in power (and performance) is probable. The detergents designed to clean the cylinder walls after compression ignition (diesel) can have a damaging effect on a gasoline engine.
In short, use the specific type and viscosity (or better) that your owner’s manual specifies…especially during the warranty period! The only deviation that I recommend is to use synthetic oil even if the manual doesn’t require it. I bought a Honda Civic new in 1993 and put 187k very hard miles on it over a 38-month (3yr 2mo) period. I used Castrol Syntec and changed the oil every 10-12k miles (the owner’s manual specified every 5k miles, the dealer said every 3k miles). Before selling it with 187k miles, I had an engine compression test performed to determine how well that car held up. Even with 187k, the engine still met the test specs for a new engine! Synthetic oil has been very good to me and my cars!
A final note…I’ve read a few comments regarding compression and the performance characteristics of diesel engines. Diesels operate at a compression ratio more than double that of a gasoline engine. Because they experience much greater internal pressure than a gas engine, diesels are typically built to a much stronger standard. That is why many diesels have a reputation of lasting far beyond 200k or even 300k miles! Diesels are also slower revving (rpm increase at a slower rate) than a gas engine and the redline (maximum safe rpm) is often much lower. Both of my current vehicles have a 6500rpm redline, but I’ve seen many diesels with the redline set at 5000rpm or less.
modern diesels have catalytic converters; there’s a two-way Diesel Oxidation Catalyst and an NOx selective reduction catalyst. The latest (US) spec for diesel engine oil is API CJ-4, and there’s no reason to believe it has any more ZDDP than motor oils for gas engines.
http://www.apicj-4.org/faqs.html
phosphorus and sulfur “poison” the catalysts, and oil for both gas engines and diesel engines have been steadily reducing the amount of ZDDP additives.
As far as the “or better” part:
For the API rating, this works. For example, if the manual calls for SL rated oil, SM or SN (newer and better formulas) is fine.
For the viscosity rating, it’s not clear-cut what – if anything – is better. If you’re not an tribologist (lubrication specialist) or automotive engineer, best stick with what’s recommended. That said, the occasional top-up or even complete oil change with a different viscosity isn’t going to ruin the engine, but consistent use of non-recommended viscosities can shorten its life.
The complete oil change with a badly wrong viscosity can ruin it.
For example, a small engine vehicle like a Smart car may require 0W30 … The 0 means it runs like water. The drainage holes in the head may be so small, that 20W50 won’t run through them ! Or the slightest bit of gluggy gunk blocks them.
5W30 won’t be a problem , while 10W40 won’t be a risk, but for the need to avoid it getting old and gluggy. Change it on time both in terms of time and mileage. Its the sum of all evils that get you…
Back to OP. Some oils are rated with both petrol (SL ) and diesel ratings.
no, it just means it’s at the bottom of the API viscosity scale.
Depends on the engine and the oil. Most N. American oils conform to the API standards, which are 2 letter codes starting with “S” for spark ignition or “C” for compression ignition, which neatly sidesteps issues with things like E85 or B10.
Each letter is backward compatible for the most part, and supersedes the previous ones. Each vehicle has a a specified code “or better” that it’s approved to use. So for example, my pickup truck is a 2005, so it’s rated for SM or better oils, meaning that I can use SM if I can find them, or the current SN, or SO whenever it comes out. Diesels work the same way. There are also manufacturer-specific specifications as well- GM’s DEXOS ratings (1 for gas, 2 for diesel), VW 502/504/507, etc…
Some oils carry multiple ratings though- for example, Shell Rotella T Triple in 10w-30 is rated as follows:
[API CJ-4, CI-4 PLUS, CI-4, CH-4, CF, SM, SL, SJ, SH; Caterpillar ECF-2, ECF-3; Ford WSS-M2C171-E](API CJ-4, CI-4 PLUS, CI-4, CH-4, CF, SM, SL, SJ, SH; Caterpillar ECF-2, ECF-3; Ford WSS-M2C171-E)
meaning for any vehicle demanding 10w-30, it’s good for any gas car manufactured from 2010 or before (SM is 2010 and earlier model years), and for any diesel requiring an API-certified diesel oil, as it’s CJ-4 rated, meaning present-day or older. It’s also certified for any Caterpillar vehicles requiring ECF-2 or 3 or Ford WSS-M2C171-E.
FYI - I was told to use Chevron DELO 400 LE diesel engine oil in my older cars/portable gas engines - lawn mower, etc. As it STILL HAS a high level of ZDDP in it…
After reading these threads here, I called Chevron to ask if this oil still has a high level of ZDDP. They said yes, it has 1300 PPM of ZDDP. And this can safely be used on older flat tappet engines.
Now if they reduce that, then the safest would be to use synthetic oil. But my engine re-builder told me to use either synthetic or Chevron DELO 400 LE.
SAE viscosity grades of the xxWxx format tell you that the oil has two viscosity ratings: the number before the W is the cold weather (“winter”) rating and the number after is the rating at operating temperature. A larger number implies a higher viscosity; SAE 0 has a viscosity of about 3.8 mm/s which is about four times as viscous as water and comparable to kerosene.
Multigrade oils are necessary to ensure oil can quickly get into all engine passages at engine startup without losing lubrication properties at higher temps.
You have reminded me what higher RPMs had to do with this, it’s not that diesels rev higher than gasoline engines, they are revving higher in autos than they used to in the old chuggers, getting more HP out of smaller engines. That requires closer tolerances in the engine, better grades of oil, and IIRC more frequent oil changes.
My absolute favorite oil to use on my bikes is Shell Rotella T (5W40) synthetic. It is a heavy duty diesel oil, but I use it for everything. My race bikes, street bikes, and dirt bikes all have Shell Rotella T in them.
Also, the SAE grades are really bands; for example, SAE 30 denotes an oil with a viscosity of 9.3-12.49 centistokes (kinematic viscosity) at 100C. This is entirely separate from the “W” viscosity.
The winter viscosities are a bit more interesting- a “W” oil, for example, has to have a minimum viscosity at 100C, and has to meet certain cranking/pumping viscosity requirements at certain temps, which vary by temperature, and are absolute viscosity, measured in centipoise(cP).
So a 10w oil has to have a minimum viscosity of 4.1 cSt at 100C, and cranking visicosity of 7000cP at -25C and a pumping viscosity of 60 000 cP at -30C.
Meanwhile, a 0w oil has to have a minimum viscosity of 3.8 cSt at 100C, cranking visicosity of 6200 cP at -35C and a pumping viscosity of 60 000 cP at -40C.
So as you can see, a 0w-30 isn’t “thinner” than a 10w-30 in any meaningful sense, except in the winter, when you want it to be thinner. At operating temperature in the engine, they’re identical, so you’re always better off with the lowest ‘w’ viscosity you can find.
In general, engines have a band of acceptable operating temp viscosities that will work in the engine, and they’re all pretty low- even SAE 40 oil is only like a maximum of 15 cSt at operating temp. Anything thicker is slower to get to the bearings, harder to pump, and generally less effective. The catch is that at say… 75 F, even a 0w oil is something like 40 cSt, with straight SAE 30 being more like 250 cSt. So even there, you still want as thin as you can find- 40 is better than 250, even if it’s a little more than 3x the preferred viscosity for the engine.
So in theory, you generally want the second number to be whatever your manufacturer likes, and the first number (the “w” number) to be as low as you can find. Unless you live in very, very cold places like Alaska or N. Dakota, there’s precious little difference in practice between say… 0w and 5w, or between 5w and 10w. So I wouldn’t go out of your way and get synthetic 0w-30 for a 5w-30 spec’ed vehicle because it’s the only way to get a 0w oil, unless you live in Minot or something. The 5w will do just fine.