Why does my car burn more oil with 5W50 full synth than with 5W40 blend?

[kilou]

Hi,

my car has now 240'000km so it's quite old but the engine runs still very fine. For the previous oil change, I have used Mobil1 5W50 100% synthetic oil thinking that the higher 50 grade would be good for my old engine. However despite this high grade the oil consumption was higher (not scary though) than it was with the oil I used previously (Motorex 5W40 synthetic blend).

I've heard it is known that when switching from synthetic blend to full synthetic, the oil consumption is supposed to grow but why? I understood this as the synthetic oil cleaning the engine and removing sludge deposits. However I did not see any leaks when using the 100% synthetic oil so the oil has not removed all the sludge that could be on the seals. Moreover why would a cleaning process make the engine to burn more oil? And why would this happen with an oil that is a 50 grade?

Now I switched back to the 5W40 synthetic blend and the engine does not eat oil anymore.

Any clue?

 

[JHZR2]

The wider the spread, the harder it is to achieve an oil with those characteristics, even in a full syn oil.

Id say that the blend utilizes more lighter ends, with more VII, and as a result the lighter stuff will be more likely to burn off. The lower spread 5w-40 may be able to be achieved without any improvers, or less and a different mix.

Sometimes engines will use some syn oil for the first few changes, as it cleans and changes how the intrernals of the engine are coated with junk. This is common in the older MB diesels, where there will be some consumption for 10-20k miles before it goes back to 0.

JMH

 

[Dr. T]

Yes, the cleaning process will/can cause you to consume more oil.

That said, Mobil oils have to be the most volatile synthetics out there. So it could simply be the fact that you're using M-1 which is known to be consumed in greater numbers in certain engines than other brands of the same viscosity.

I would either do another interval or try another brand of 5-50 and report back to us.

 

[Bob Miller]

Its also a possibility that the higher viscosity oil remains on the cylinder walls and burns. The riungs may not be scraping the oil off the walls.

Check for sludge as well.

 

[Triple_Se7en]

Now I switched back to the 5W40 synthetic blend and the engine does not eat oil anymore.

Sounds like your vehicle likes the 5W-40 Motorex Blend. Why not beef up the oil by substituting one quart of synthetic blend with one quart of full synthetic instead? If no full synthetic is available in 5W-40, it won't hurt to substitute a single quart with either 10W-40 or 10W-30.

 

[moribundman]

I think the full synthetic oil squeezes by the rings more than a non-synthetic.

 

[oilyriser]

My experience when changing a car to synthetic is that consumption occurs at first, then stops. It might have something to do with different seal swell additives taking time to start working, but that's just a guess.

 

[kilou]

Thanks for all your replies! Actually I'm very satisfied with the Motorex 5W40 I use now although this is a synthetic blend. I only wanted to use a 100% synthetic oil such as the Mobil1 5W50 that I can get for dirty cheap under another name (Carrefour) in France. However this was only because I heard so much praise concerning synthetic engine oil that I did that switch but I don't know if this is really worth it.

I'll probably try again the 5W50 this summer (I still have 7 liters left) but I'm really pleased with the Motorex. Honestly I do not know if it is so important to run a full synthetic versus a synthetic blend even in the long run. Do you really think this will make a difference when it comes to engine durability, sludge deposits etc??

PS: I also think that my engine was noisier with the Mobil1 than it is with the Motorex again despite the M1 was a 50 grade. This is rather subjective but I've also heard other people mentionning noisy engines with M1 oils.... Also as you said, the 5W50 is a wide spread that can be achieved only with viscosity improvers so maybe a 100% synthetic 5W50 is not better than a 5W40 synthetic blend in that respect. What do you think?

 

[Dr. T]

No, there's nothing wrong with the 5-50. You've just confirmed the noise and consumption issuse seen with M-1 in some engines.

For eg. my car consumes more M-1 15-50 than Castrol Syntec 5-50. Larger spread VI's in the 15-50? Don't think so.

 

[Cool_jazz]

quote:

---

Originally posted by kilou:
Hi,

my car has now 240'000km so it's quite old but the engine runs still very fine. For the previous oil change, I have used Mobil1 5W50 100% synthetic oil thinking that the higher 50 grade would be good for my old engine. However despite this high grade the oil consumption was higher (not scary though) than it was with the oil I used previously (Motorex 5W40 synthetic blend).

I've heard it is known that when switching from synthetic blend to full synthetic, the oil consumption is supposed to grow but why? I understood this as the synthetic oil cleaning the engine and removing sludge deposits. However I did not see any leaks when using the 100% synthetic oil so the oil has not removed all the sludge that could be on the seals. Moreover why would a cleaning process make the engine to burn more oil? And why would this happen with an oil that is a 50 grade?

Now I switched back to the 5W40 synthetic blend and the engine does not eat oil anymore.

Any clue?

---

Hey, you are not alone. I am driving the Alfa Romeo 156 2.0TS and I noticed that my car "drinks" Mobil 1 5w50.

I used to be on 5w40 (initially TOTAL Quartz 5w40 and subsequently Agip Extra HTS 5w40) and my car only consumes 0.5L of engine oil per 5,000km. But it consumed 1.0L of Mobil 1 5w50 every 5,000km. Now I am back to Agip Extra HTS 5w40. IMHO Mobil 1 does not work in my car...

By the way I tend to drive quite hard. The first 2 gears sees the redline everyday and the 3rd gear about once a week.