Why such frequent oil changes?

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Oscar
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Why such frequent oil changes?

Post by Oscar » Wed Feb 01, 2006 2:09 pm

I was chatting to someone the other day who asked why we need to change diesel engine oil every 5k miles. I realised that I don't know - I've always just taken it as read.

Can anyone explain?
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Post by DLM » Wed Feb 01, 2006 3:09 pm

Diesels soot up their oil very quickly: that's one thing. There will be others....hopefully along in a minute.
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Post by DavidRutherford » Wed Feb 01, 2006 3:12 pm

there's nothing that odd about 5k oil changes. After all, many years ago, all cars had a 3k oil change interval.

Oil wears in 2 main ways: it gets diluted with carbon, soot, acids and condensation, and the molecules get broken up, thus reducing it's ability to lubricate.

As I understand it, the risk posed by used oil is mainly to do with these various odd-lengths of molecule in the oil

5k miles is a trade off.. it's the oil's ability to stop it's molecules being broken up, and it's ability to hold carbon soot and acids in suspension, ready for draining. Higher quality oils, including fully synthetics can have massively longer drain intervals.

The other reason for "short" oil change intervals is the fuel itself. Nasty stuff really, and some products of combustion will always end up in the oil. It's very noticeable that if you run a diesel engine exclusively on vegetable oil, it doesn't blacken its oil anywhere near as quickly. Same true of a petrol engine. Run it on LPG, and the oil stays clean for ages.

If you go looking for it, there's masses of information about this on the web (some of which is erronious, so be careful!)
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Post by Oscar » Wed Feb 01, 2006 4:54 pm

Excellent replies DLM and David. Not really interested in researching it further, but good to know the main reasons. Thanks

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Post by Kitch » Wed Feb 01, 2006 6:34 pm

The 16v's have a 6000mile interval too.

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Post by Stewart (oily!) » Wed Feb 01, 2006 7:09 pm

When I got my first ever diesel car a work acquaintance who ran a fleet of trucks for many years told me to change the oil every 3000 and keep an eagle eye on the cooling system "and if you ever overheat it, sell it" apart from doing a few headgaskets I followed his advice rather closely, i have also had experience with heavy plant where an engine rebuild can cost upwards of 30K, these vehicles are invariably driven flat out for 70 hours a week by guys who were never top of the smarts pile, oil changes were frequent and engine breakdowns rare, my old 1769TD lump had over 300000 miles on it when dismantled and the honing marks were visible on the bores, my concession to turbodiesel motoring is to use a better quality oil and to change it at 5k intervals instead of cheaper oil at 3k. I also work on marine deisels where the enemy is lack of use, cold damp and condensation, the cure is again frequent oil and filter changes.
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Post by M » Wed Feb 01, 2006 10:30 pm

I used to drive cars with the A-Series engines by BLMC - never let the oil go over 4K miles due to it being mashed up by the gearbox in sump.

Habit (and my dad) mean I change oil at 5K miles now - regardless what the manufacturer says....

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Post by DavidRutherford » Wed Feb 01, 2006 11:45 pm

And lets face it, what's £10-£15's worth of oil and filter over 5000 miles worth of fuel? absolutely nothing, and is very good preventative maintainance.
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Post by AlanS » Wed Feb 01, 2006 11:51 pm

I'll probably horrify all of you, but I rarely let mine go beyond 5000klms.
I work on the theory that the cheapest thing I'm ever going to do to any of my cars is change the oil.
I suppose having worked as long as I did in a used car yard and seeing the results of prolonged oil changes and having worked on so many cars that have what appears to be treacle in the sump, I'm more fussy than most.
I just changed the oil on my Xantia which I had overlooked and did about 12,000 klms on and discovered that when I put it into an old container, it looked as clean as the average 20W50 mineral when new.
I also had a batch of synthetic that I'd put into an old container that was so clean that my son grabbed it (clean container mind you) and put it into his engine when he was doing an oil change. Didn't have the heart to tell him at the time, but a couple of weeks later I did ask where my old sump oil had gone!! :shock: He's never grabbed one of my cans of oil since. (It's only a Jap car so it doesn't rally matter. :twisted:
Whenever I have to take anything off one of my engines, say a rocker cover for example, everyone always comments on how clean my engines are internally.


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Post by DavidRutherford » Wed Feb 01, 2006 11:58 pm

While we're on the subject, to keep the guts of my engines from being the usual black gooey mess that diesels are, I tend to run a flush every other oil change too... but I don't use flushing oil, as it's a) expensive and b) bad for the engine if any remains in there. So instead, I run 2 litres of normal engine oil round at no more than idle for 5 minutes. It's just enough oil to make sure the pickup is always submerged, and there's always oil pressure, but not enough to actually drive on (it would overheat in no time at all)

Draining that back out again usually clears a fair bit more black, and means that the fresh oil tends to stay cleaner for much longer. Probably doesn't make a massive difference to engine wear etc., but as it costs me about £2 to do, I think it's worth it. Certainly the camshaft looks an awful lot cleaner than many other XUD engines I've seen. (and owned for that matter)
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Oil changes.

Post by Geoffrey Gould » Thu Feb 02, 2006 12:19 am

Hi being somewhat old fashioned, due to my fathers strict 2000 mile change without fail program, I seem to change engine oil at no more than 3000 mile intervals. my son does the same and his BX diesel is up to 380,000. The honing marks are still visable in the bores.
Used to change the oils at 3000 mile intervals on a fleet of vehicles that most had done between 100,000 and 300,000 miles with no problems at all. Actually I lie, a Bedford at 35 years old running the original engine (not bored or anything) distroyed the cam followers and took the block with it.
Look after the engine and keep it clean and it will look after you.
Cheers.
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Post by Oscar » Thu Feb 02, 2006 5:39 pm

Yes I've noticed recently that the old oil I am changing is becoming cleaner at every interval. Think I'll try David's trick next time.
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Post by jeremy » Thu Feb 02, 2006 6:19 pm

Is it worth it? For most of my motoring career I haven't really bothered about oil changes as the thing was more likely to rust away than sieze up! I've had my share of engine trouble but its usually valves so that's not really lubrication attributable.

Part of my reasoning has always been that most have burnt or leaked so much that the entire contents of the sump had been changed well before 5000 miles. As regards the eventual life expectancy of the engine - well as I have said - due to the effects of corrosion it was unlikely to be a problem for anyone.

I do change our Citroens regularily - partly out of respect for my TD which has done 200,000+ and still uses no more than our 77,000 mile ZX (both about 1 litre between changes)

Will I ever see the benefit on the TD BX? - I may be seeing it now!

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Post by Stewart (oily!) » Thu Feb 02, 2006 6:52 pm

My old motor vehicle lecturer would say if the oil is staying clean it could be because it might not be picking stuff up :twisted:
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Post by Mr B » Thu Feb 02, 2006 11:56 pm