Visc 40C - cSt

[Vulusmaximus]

Hi guys, a relative noob to oil analysis question for the hive mind.

I've had several UOAs conducted (BMW N57 diesel engine) and I've seen a number that people have uploaded as well. The UOAs comment on the oil chemistry against a Visc 40C - cSt metric. I've been searching around and I'm having difficulty understanding the significance of the numbers that are being reported.

1st result: BMW Oil 0w/30 58.0 (after 7,151 miles)
2nd result: BMW Oil 5w/30 55.0 (after 789 miles)
3rd result: MOTUL 5w/40 66.8 (after 333 miles)
4th result: MOTUL 5w/40 66.7 (after 2,384 miles)

Looking against the Technical datasheet for the MOTUL oil it says 'Viscosity at 40°C (104°F) ASTM D445 77.1 mm²/s'
https://www.clubmotul.co.uk/p/8100-xclean-gen2-5w40-fully-synthetic-engine-oil/33

How do I understand what is a good result and what is a bad result ?

Thanks for your help

Kind regards to all
Ian

 

[Rand]

That is the viscosity at 40 degrees Celsius and not really a significant number.
There can also be small differences in measuring.

Viscosity at 100 degrees Celsius is used to determine if it stayed in grade among other things.

 

[JavierH19]

Could you post the uoa's on here. Can't get anything out of just the 40cst value.

 

[twX]

cST and mm^2/s are the same units. The KV40 of the Motul dropped from 77 cST to 67 cST, due to a combination of shear-thinning and fuel dilution. If all of the thinning was from fuel dilution, a 13% drop in KV40 would indicate roughly 4% 3.5% fuel dilution with diesel. There would have been some shear thinning as well though, so fuel dilution would be lower than that. I wouldn't be concerned about the reduction in viscosity.

Oil will eventually start to thicken up from oxidation, and from soot contamination on diesel engines, but these things don't happen within 2300 miles, so the viscosity change in this case should be purely from shear and fuel dilution.

Edit:
Fuel dilution will have a greater thinning effect on the KV40 measurement than it would for KV100. A 13% drop in viscosity might seem a bit high, but there would be less of a drop in KV100 or HTHS150.

 

[Jetronic]

That is the viscosity at 40 degrees Celsius and not really a significant number.
There can also be small differences in measuring.

Viscosity at 100 degrees Celsius is used to determine if it stayed in grade among other things.

Dn't forget differences in blending. I would consider +/-10% spot on

 

[Vulusmaximus]

Could you post the uoa's on here. Can't get anything out of just the 40cst value.

Here's the raw data data from the last 4 tests and the same data set extrapolated to 10,000 miles. Interesting to see that the ppm rate for the Iron and Ali and Sodium is much improved with the MOTUL 5w/40 oil, although currently still too high. I'm going to resample again when the car gets to 54,200 (and also change the oil as well). Note that at the last change at 49,208 I had the engine flushed.

Thoughts, observations... ?

 

[Rand]

You cant extrapolate anything that would be total fiction.

 

[Vulusmaximus]

You cant extrapolate anything that would be total fiction.

Some figures can't be extrapolated, but some can. For instance without extrapolating you have no idea if a ppm figure is good or bad. The problems come when extrapolating from a small mileage, that's obviously open to be skewed especially with this engine when not all the oil can be drained at the oil change if you have the X-Drive version of the car.