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The car: BMW '90 E30 320i Cabrio with the six-cylinder M20 engine.

The problem: the red engine oil warning light inside the instrument cluster (not the check control) lights up after some driving. But the engine sounds and works just fine. Turning off the engine and restarting causes the light to go off. But it comes again after some cruising. I cannot say for sure but it seems like this happens rather on hot days.

I had the engine oil changed about 6 weeks ago but to be honest I didn't pay enough attention and I'm not sure which grade the mechanic put in. The problem with the warning light started about two weeks ago.

The BMW is a weekend driver and does not get moved daily.

What could be the most likely cause for this problem? Oil pressure sensor? Motor oil too thin? Something more serious?

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    You have probably done this, but check your oil levels or you may end up with a seized engine. The "red engine oil" light is a a warning that oil pressure is too low, which is usually caused by lack of oil. – Wulfhart May 21 '12 at 17:19
  • @Wulfhart, you should repost this as an answer - it's almost certainly the issue. – Bob Cross♦ May 21 '12 at 19:14
  • I bet it's just too thin an oil and running the car on hot days lowers oil pressure enough for the warning light to come on. Or the relief valve on the oil pump is stuck open. It depends on the car, but the "oil warning light" can mean anything from "oh shit your engine is about to seize up in 5 4 3" to "your oil pressure is slightly lower than we would expect under normal circumstances." In my experience, almost all engines give the warning at dangerously low pressures. – Jim W Feb 22 '16 at 16:39
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As other people have already pointed out, the warning light is the oil pressure warning light so you really need to get to the bottom of this, especially if the light comes on at any other engine speed than idle.

I think the culprits - especially if the problem started shortly after an oil change - could be (in decreasing order of likelyhood):

  • Oil viscosity. If whoever changed the oil just poured 5W30 in because that's what they have on the shelf and it's cheap, that's most likely the wrong oil viscosity. In those engines I would put at least 10W40 in, if you're in a hot climate you might want to go with a higher viscosity. Actually this page recommends 20W50 for an M20, which is a recommendation I've heard before. If that's what's supposed to be in the engine and it's got 5W30 in it, that can certainly trigger the low oil pressure warning light.
  • Cheap crappy oil filter, or they "forgot" to change the oil filter. Either can cause an oil pressure drop that can show up at idle.
  • Sender bad. If it was a Fiat, Alfa or Ferrari that would be my guess :). In this case, it's a bit more unlikely but it happens.
  • Overall wear on the engine or the oil pump getting worn is resulting in low oil pressure. Only way to determine that with reasonable certainty is to use a properly calibrated oil pressure gauge - your mechanic of least distrust should be able to check this for you for very little money. It's probably worth doing.
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  • thanks a lot for the great and informative answer. The idle pressure is fine. I've noticed that the oil pressure light is likely to go on and stay so after a burst of acceleration. When I start the engine, the light goes off quickly. I'll change the oil and the filter as soon as I can and hope it's not the pump. – Engin Kurutepe May 22 '12 at 17:07
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You have probably already done this, but check your oil levels or you may end up with a seized engine.

The "red engine oil" light is a a warning that oil pressure is too low, which is usually caused by lack of oil.

Brian Knoblauch in the comments below points out that a faulty oil pump can also cause the oil pressure to drop.

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    I found out the hard way that an oil pump can also fail in a manner where it just supplies greatly reduced oil pressure. In my case it was just barely supplying enough to keep the light off in normal driving. On really hot days sometimes the light would flicker while idling after starting. I could hear no untoward sounds at all, but when I went in for the 180k timing belt change I had them borescope the engine. The cylinder bores were trashed. Dropped the oil pan and the main bearings were shot too. Get the oil pressure checked ASAP! – Brian Knoblauch May 21 '12 at 21:07
  • thanks for your answer. checking the oil level was the first thing I did and it's ok. – Engin Kurutepe May 22 '12 at 7:34
  • @BrianKnoblauch, oh look, another answer framed as a comment! ;-) Wulfhart, you might want to incorporate Brian's suggestion into your own. – Bob Cross♦ May 22 '12 at 13:09
  • The asker stated that it is instrument cluster oil warning, not so-called “check control”. The latter is the little panel with LEDs (near windscreen rear-view mirror on mine) which light up if engine is low on oil, coolant, if brake lights do not work etc. So, given all the senders are working properly, it is not an oil level issue. Obviously, it is easy to check with a dipstick. – theUg May 23 '12 at 18:19

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