I have a 04 Scenic Expression DCI 80 with 65,000 miles on the clock. The car runs fine with no problems. I am thinking of giving the car an engine flush/oil treatment. Is this a good idea or could it cause any damage or blockages to the engine or turbo.
Well, opinions seem divided on the use of engine flushes and/or 'treatments'. Some say there is a risk that old oil 'sludge' can be lifted and deposited in other areas - like the turbo feed.
Personally, if the engine is running well and has a known history of on-schedule oil changes then I wouldn't bother. You can shorten the oil change interval if you like - I'm not a great believer in 'extended drain intervals', no matter how good the oil. But maybe I'm just old-fashioned
I think a lot of the trouble stems from manufacturers' desires to reduce servicing costs (particularly for fleet users) by increasing service intervals.
Whilst I accept that better engine oils are available than 10 or 20 years ago, I prefer to pay a tiny bit more for 5 litres of oil and a filter - and change the oil twice as frequently as that 'recommended'.
The cost pales into insignificance next to the cost of sorting out major engine damage
Here is my reccomendation:
As others have said you don't really want to use flush, but use a high quality oil and filter (making sure you allow plenty of time for the old oil to drain out fully, at least 20 min pref couple of hour), also, change the filter at the midpoint of the service schedule which makes a big difference to the cleanliness of the oil, reducing the amount of soot and other crap it has to suspend, which in turn allows it to perform it job much better.
12k/12month not 18k/24month is an ideal service interval, with filter change at 6k/6month. A filter costs a few squid so is much cheaper than replacing the oil, add to that that a high quality modern fully synth oil can handle 12month/12k without effort, so is a waste to change on the olde 6month/k schedule.
If going for the fully synthetic 5-40 oil which is reccommended by renault for this engine, don't be tempted by the cheap ones, they are not actually genuine synthetic, just mineral that has undergone a certain process allowing it to legally be called 'synthetic', and is therefore really a semi synthetic.
I'd reccommend opieoils.co.uk, and go for something like the motul 8100 xcess or mobil super 3000 x1 which meet good specs and performance levels and are ideal for this engine. If budget is very tight however, the fuchs supersyn should suffice quite nicely.
I se your point about changing the oil filter to try and remove the carbon particulates from the oil itself but sadly it's unlikely to prolong the lubricating or cooling properties of the oil in itself.
Engine oils tend to break down due to chemical contamination from the combustion process rather than contamination by carbon particles. But the oil filter is incapable of filtering out the chemical contamination.
I still think changing the oil filter more often is a good idea. I am only making this comment to emphasise that whilst an oil may look clean it doesn't necessarily mean it isn't contaminated or has retained it necessary qualities.
A cheaper 5w-40 synthetic may work almost as well as a brand name one for a while so it depends on the cost of your time vs the cost of the oil.
If you can change oil every 6 months or 3000 miles or similar then your oil will be cleaner on average than if you changed just once a year with a more expensive oil.
If you dont change oil very often then definately choose a good oil.
a top oil in the second half of its usable life in the engine could be no better than a cheap oil thats quite new. If you go thru a lot of oil maybe a 20liter container from a wholsale seller is a good buy.
I usually buy a good brand when I see it on special somewhere so I allways have enough for a change or 2 sitting at home.
What you have to bear in mind though is that a High quality, High spec -genuine synthetic- has to comply to many manufacturer extended service intervals of up to 36month/36k and is engineered to cope with all the acids soots and other by-products for this length of time, but also a -proper- synthetic base stock degrades -significantly- less than a mineral base stock even if it has been hydrocracked to be called synthetic.
A hydrocracked mineral 'synthetic' will degrade relatively rapidly compared to an oil with a synthetic, manufactured base stock.
It is for this reason you will only find the higher specs being met by top end genuine synthetics as a mineral-synthetic base cannot.
As said a faux synthetic may lubricate quite well initially, and if changed every few month would be ok, but this is a false economy, a cheap 'synth' costs around £18-20, but a high end oil is about £35-45 (per 5L) which comes back to my previous point, an oil with a proper synth base stock engineered to handle the byproducts of an engine for 2-3year/24-36k will have barely degraded at all (due to the base stock) over 12k/12month and would be a waste to change earlier, the biggest issue for the oil and engine within this period is soot and other particulates, which is why the filter change at midpoint.
The other issue is that a synthetic base oil has an exponentially higher shear strength of the oil film, it is this film that protects the bearings from the sudden and extremely high impact loads generated by the pulsation of the pistons via the conrods, lobes of the camshaft, and so on. It is these brief forces that can cut through the oil film and enable metal to metal contact which is where most of the wear occurs. The synthetic base stock has a huge resistance to these forces when compared to a hydrocracked mineral base stock, and therefore provides much higher protection from wear, and also far better efficiency from the engine. Soot causes trouble in this area as it prevents the oil from forming as much of or as thick of a 'chain', and also will itself wear the components making a friction link from bearing to bearing through the oil, its not as bad as metal to metal contact, but metal to carbon isn't good either.
Another note is the insulation properties of carbon, this is a great deal of what helps keep the oil hot, reducing its vital cooling effect.
It has been proven (though I can't for the life of me remember what or where the document was that I was reading) that changing the filter frequently has a significant effect on wear protection and efficiency of the engine than the degradation of the oil itself, within the actual natural lifespan of the oil whilst it is in use. One of the things in the article stated about a motorsport team who had this idea, and rather than changing both the oil and filter after each race, they just changed the filter. They found that the engines stayed just as reliable, were still in top condition when they were stripped, and they got four to five (iirc) races out of one fill of oil, saving both money and time to no ill effect. They did monitor the conditon of the oil itself too.
Btw the above post was for information and to explain where I was coming from with what I had posted for recommendation.
It was not meant to be preachy, patronising or any other such thing.
Neither am I an expert oil chemist, just trying to share the knowledge I do have in relative laymans terms to help anyone who didn't know any of it.