Basic oil question

E36 M3oil The BMW spec is 15W40 non-synthetic.
Browsing my local Auto parts store, whose computer confirms the grade for my E36, all the 15W40 oils on the shelf are labelled "for diesel
engines". I shuddered at the prospect of putting the "wrong" oil in an M3, so I just went and bought a case from a BMW dealer. At less than $3 a quart I will probably do the same again. But I'm curious.
Other than the additives that oil manufacturers use to claim superiority over rivals, are oils with the same SAE classification basically the same?
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mcquarrie wrote:

Yes.
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on about something:

I believe the diesel oils are actually a little tougher. They need to resist the breaking down when exposed to diesel. Petrol is less of a problem as it evaporates from the hot oil and doesn't mix with it.
Dodgy.
--
MUSHROOMS ARE THE OPIATE OF THE MOOSES

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Dodgy warbled on about:

You're partly correct. Diesel rated oils do have higher ratings for extreme pressure (which protects when the oil-film goes away in a bearing..)
It's not the difference in fuel that causes the higher-sheer-breakdown rating of diesel rated oil - it's the higher compression ratio (and the actually detonation pressure)in a diesel engine which causes a higher force on the tie-rod bearings. There shouldn't be any diesel oil getting into the sump oil unless the engine has a hole in a piston..
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No they are certainly not. The SAE standard is only a measure of viscosity. The API standard which you should also have a value recommendation for your car, is the measure of performance and quality. Also BMW might stipulate their own higher standard as being compulsory along with the API standard. Newer BMW will need an oil meeting ACEA A2 or A3 or even BMW LL98 or newer. BMW LL is a long life oil for use with their nominal 15000 mile service schedule so it does not apply to your car. As I understand it, your car is an M3, so depending on climate you would need either a 10w/40 or 15w/50 synthetic Mobil1. I know it might not actually need synthetic but you have already found that most 15w/40 oils are diesel specific although if you search you will easily find ones that are 'multifleet', that is they carry a combined diesel and petrol rating- commonly APICH4 and SL. Have a further look at what your car needs and the appropriate ratings on the can and get them to match. The SAE viscosity is only a small part of the equation.
Don't worry, while it is maybe not as simple as you thought, choosing an oil is not rocket science either.
Huw
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mcquarrie wrote:

As someone already said, it is the API classification that you need to pau attention to as well. However, I beleive most of the 15W40 oils intended for diesel engine use are as good or better than the stuff BMW sells.
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-Fred W

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Like others here already said, check the other requirements and make sure the oil meets or exceeds them. I think you are safe though. I have been using oil rated for diesels in my motorcycle for years and so have many friends. The oil is the shell rotella-T It's synthetic and cheapest at wal mart by the gallon.
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