“Diesel Oil” for E36 M3?

[Duck360198]

Hey everyone. Well, I was a bit surprised when I had a local shop suggest diesel oil for the E36 M3. But, the shop does a great job and has quite a bit of experience so I don’t feel like I should question it...but here I am. When I asked about it I got the following response:

“That is the oil we have always ran in the older cars. The factory fill from BMW is no longer available. 15W-40 as required for the older BMWs, in a synthetic, is next to impossible to find. All oil lubricates. The difference in oils marketed to be used in gasoline engines, marine engines, diesels are all in the additives mixed in with the oil itself. These additives have a lot of overlap between the oils. Nomenclature calling an oil “diesel oil” or “gasoline oil” is strictly marketing and branding.”

Maybe I’m caught up in the nomenclature after all. It sounds like the response has merit. I dunno...thoughts?

[K-Dawg]

Meh, don't overthink it. The S52 doesn't tend to suffer many oil related failures. Oil lubricates, but not all oil is the same. It would take some strong evidence or some special circumstance to convince me that I should run something other than the manufacturer recommend oil spec in any engine. For the S52, that's a 5W-30 or 15W-40 API Sx, depending on ambient temps.

Supposedly diesel engine oils can damage the emissions equipment on gas engine cars. What are you looking to achieve by switching to a diesel engine oil?

[Duck360198]

Meh, don't overthink it. The S52 doesn't tend to suffer many oil related failures. Oil lubricates, but not all oil is the same. It would take some strong evidence or some special circumstance to convince me that I should run something other than the manufacturer recommend oil spec in any engine. For the S52, that's a 5W-30 or 15W-40 API Sx, depending on ambient temps.

Supposedly diesel engine oils can damage the emissions equipment on gas engine cars. What are you looking to achieve by switching to a diesel engine oil?

It’s why I questioned the shop in the first place. I didn’t understand the recommendation by the shop initially, but their explanation seemed to make some sense. I guess more than anything I was curious if people disagreed with the shop’s thoughts.

[EthanolTurbo]

It’s why I questioned the shop in the first place. I didn’t understand the recommendation by the shop initially, but their explanation seemed to make some sense. I guess more than anything I was curious if people disagreed with the shop’s thoughts.

I would run diesel oil if you're using E85 as the detergents in diesel oil will slow down the breakdown/degradation of oil due to ethanol. If you're not running E85 you'd be fine with 10w-30 or 5w-40.

[IamFODI]

Not sure what the origin of this myth was. What I know is that around 20 years ago, people on various forums (mainly BITOG I think) fell in love with the idea of using Shell Rotella T6 in gas engines. The Rotella oils are for Diesel engines. T6 is a fully synthetic version. It could be purchased basically anywhere and at a lower price than many full-synthetic gas engine oils. Thus, people saw it as a kind of holy grail: cost-effective, widely available, and fully synthetic with a heavy duty formulation.

People posted impressive looking oil analysis results from T6 in their gas engines. The oil held up well and wear metal numbers were low. People saw this as validation.

Since then, the idea of running a Diesel oil in a gas engine has taken on a life of its own.

All of this is based on a few key myths and misconceptions:

1. Diesel oils aren't categorically "tougher" than engine oils. They are sometimes higher in certain additives, but not always -- especially now that they, too, have to be formulated for sensitive emissions equipment (e.g. DPFs). And even when they are, their additive packages are usually optimized for different conditions because Diesel engines work differently from gas engines (hence why there are separate gas and Diesel engine oils in the first place).

2. Oil analysis isn't nearly as useful for detecting wear as people think. It has inherent limitations that few people seem to understand. Long story short, in most cases, low wear metals numbers cannot be taken to mean low wear.

3. People always tend to think higher levels of detergent/anti-wear/anti-friction additives are better, but that's not necessarily true. Too much detergent can interfere with the anti-wear chemistry. Too much anti-wear can cause deposits. Everything has to be balanced, and the right balance for one kind of engine isn't always right for another kind of engine (again, hence why there are separate gas and Diesel engine oils in the first place).

In other words, running Diesel oil in a gas engine certainly works -- but the idea that it's better than a decent gas engine oil is based on seriously flawed reasoning and poor evidence.

With that in mind...

“That is the oil we have always ran in the older cars. The factory fill from BMW is no longer available. 15W-40 as required for the older BMWs, in a synthetic, is next to impossible to find. All oil lubricates. The difference in oils marketed to be used in gasoline engines, marine engines, diesels are all in the additives mixed in with the oil itself. These additives have a lot of overlap between the oils. Nomenclature calling an oil “diesel oil” or “gasoline oil” is strictly marketing and branding.”

Is it hard to find a fully synthetic 15W-40? Yes, absolutely. Because when you're formulating a fully synthetic oil, it's trivial to get a better viscosity spec like 5W-40 or 0W-40 without significant downsides for the vast majority of applications. Very few applications are hard enough on engine oils to benefit from the marginal extra robustness of a 15W-40. The rest would see only downsides from the poor cold pumpability.

Gas, marine, and Diesel engines have different additives... but the differences are "strictly marketing and branding"? Which is it?

I'm sure the shop is right that the oil they're recommending works fine. But is a 15W-40 Diesel oil optimal for the S52? I doubt it. It almost can't be optimal, given how many great oils there are that are specifically formulated to BMW's specs. I also strongly suspect the shop's reasoning is based on some misunderstandings about engine oils.

Are they sure their oil of choice is a Diesel engine oil and not a heavy duty engine oil? People sometimes confuse the two. I know I used to.

As K-Dawg suggested, engines aren't nearly as sensitive to oils as people think. Most performance differences between oils wouldn't pan out for tens of thousands of miles, and by that point the effects would be hard to distinguish from everything else that happened to the engine in that time. Even if you had the resources to figure it out -- which, trust me, you don't -- it's extremely unlikely that any oil will do much better than what BMW has recommended (now or in the past).

The best oil for your car is... basically any oil FCP Euro lists for your car. Not because they're necessarily the "best" performance-wise, but because they're as close as you're likely to get, and almost free after the first order. There's no way any other oil can postpone your next engine rebuild long enough to make up for how much more expensive it'll be in the long run. Especially if it has an outdated viscosity spec like 15W-40, which could increase cold start wear over a 5W-40 or 0W-40.

Viscosity-wise, if you track your car (or get the flickering oil light at idle when the car is warm), you might want to get one of the xW-40s. Otherwise, any viscosity you pick will be fine.

If you really want to feel like you're going above and beyond, FCP Euro also sells Motul 300V. That oil really does have a heavy duty formulation and is made for gas engines. In some cases it'll work comparably to regular engine oil that's one grade thicker (e.g. xW-30 vs. xW-40). Again, highly unlikely to do better in the long run than the other stuff they sell, but it's a good way to scratch the itch.

If it were my E36 M3, I'd run this or this.

I would run diesel oil if you're using E85 as the detergents in diesel oil will slow down the breakdown/degradation of oil due to ethanol.

Detergents won't prevent the breakdown. What they will do is hold the breakdown byproducts in suspension so that they're less likely to form deposits, clog the filter, or abrade bearings/cylinder walls etc. And, again, that might come at some expense to the anti-wear and anti-friction properties of the oil.

[bigjae46]

Low sulfur diesel caused lubrication issues and now biodiesel. Diesel oil will have more lubricity additives in it. Will likely ruin catalytic converters.

I used Valvoline VR racing oil which has elevated levels of zinc in my 330.

I stick with the BMW OEM oil. There's probably better out there...I'd rather stick with a known quantity.

[digger]

For an e36 m3 engine pretty much any oil of suitable viscosity will be fine it’s about as sensitive to oil as an engine in a Toyota Camry

[Albino09]

They've pulled the article so I can't link it, but Blackstone Labs once released a report comparing all of the different oil brands they'd received and compared them on common engine data sets. The result of the analysis was that the Rotella T6 ("diesel oil") performed better in wear metals than every other brand.

That said, it was not a case of "all other oils are bad and only Rotella is good". Just that Rotella did very very well. In any case, you should just order whatever FCP has and take advantage of the warranty unless you hate going to the post office to ship used oil.

[digger]

If you want to compare oils for wear you run dedicated tests and look at wear of components not oil analysis

[IamFODI]

If you want to compare oils for wear you run dedicated tests and look at wear of components not oil analysis

[Albino09]

Since most of us don't possess a lab to run dedicated tests, and the OP probably isn't interested in doing bottom-end teardowns after each increment, the oil analysis is probably fair enough? Also:

That said, it was not a case of "all other oils are bad and only Rotella is good". Just that Rotella did very very well. In any case, you should just order whatever FCP has and take advantage of the warranty unless you hate going to the post office to ship used oil.

[IamFODI]

Since most of us don't possess a lab to run dedicated tests, and the OP probably isn't interested in doing bottom-end teardowns after each increment, the oil analysis is probably fair enough? Also:

It might be the only available tool, but that doesn’t mean it’ll do the job.

[Albino09]

It might be the only available tool, but that doesn’t mean it’ll do the job.

Ok bud, it wasn't my study so I'm not gonna fight a stranger on the internet over it.

[IamFODI]

Ok bud, it wasn't my study so I'm not gonna fight a stranger on the internet over it.

Last thing I wanted to do was pick a fight. Apologies if I came across that way.