I will start this post by saying that I really admire DBW, both for his knowledge and his willingness to share.
I believe (and I believe I can prove) that while part of the below is correct, there are misinterpreted results that support an incorrect conclusion.
Here are the points that will be challenged below:
1. Wear Rate is not higher in the first 5000Km - as inferred below. It is actually the lowest. Also, the more an engine oil is used, the more wear to the PRECISE and expensive parts is generated.
2. More frequent OC benefits the engine
3. UOA's are not a good indicator of CRITICAL engine wear - in the places where it really matters
4. A *very* critical aspect is filtration. Size does matter, and in my opinion, the smaller particle sizes pose a threat.
Brace yourselves for a lengthy post.
Let's start:
Drivbiwire said:
STOP STOP STOP!
[...]
VW does NOT want oil change intervals of less than 10,000 miles due to how the oils function in the engine, shorter intervals INCREASE WEAR.
Don't argue with me about it, if you take the time to track wear rates during an oil change at 250 mile intervals you can plot the reduction and stabilization of the wear rates out beyond 25,000 miles!
I believe this is misinterpreted data. Yes, wear rates (as determined via UOA's and normalized in ppm/l and 1000Km show these numbers, BUT:
Filters are a very important consideration. First of all filters... well filter - thus retain - contaminants. Therefore what the UOA tells us is what is the equilibrium level for contaminants in the oil.
To make matters more complex, filtration efficiency varies by particle size and by number of passes. In the real world (multi-pass) the numbers could be like so for a paper-type media:
40 percent capture efficiency at 10 microns,
60 percent at 20 microns,
93 percent at 30 microns, and
97 percent at 40 microns
Let's simplify by assuming an overall 60% filtration efficiency.
Contaminant (wear particles, among others) are generated differently depending on conditions - oil quality, engine regime, temperature, break-in, etc. But
let's simplify by assuming an overall constant wear generation rate - say 10ppm/l for each 1000 miles.
This means that for the first 1000 miles, there are hardly any wear particles in the new oil.
At the end of
the first 1000 miles, we would have:
Initial contaminants = 0 (fresh oil)
Generated contaminants = 10ppm/l
Retained contaminants = 60% of 10ppm/l = 6ppm/l
Free contaminants (FC) = 4ppm/l
---------------------------
2000 miles FC = 5.60 ppm/l
3000 miles FC = 6.24 ppm/l
4000 miles FC = 6.49 ppm/l
5000 miles FC = 6.59 ppm/l
See how the numbers evened out? Just like DBW stated, basing your conclusion on the UOA numbers lead to incorrect results - the wear rate was constant, yet the UOA numbers were increasing. This is NOT A SIGN OF ACCELERATED WEAR. Wear is constant in our very simple model. The increase in numbers is due to the system reaching equilibrium. It ends up at the point where the filter can retain the same amount that gets generated.
This is why IMHO, the following are incorrect:
1. The filter plays a very important role in masking WEAR RATES, as such basing a wear rate on the UOA numbers is relatively incorrect. THERE is a relationship between the UOA wear rate numbers and the engine wear rate ONLY WHEN THE SYSTEM IS AT EQUILIBRIUM.
2. The more an oil is used, the more smaller contaminants it holds. I will explain later on why IMHO, this is the really problematic aspect.
Drivbiwire said:
Anybody that tells you that short oil drain intervals are good for your motor don't know what they are talking about!
DB
This is where the problem starts; Contaminants (and by this I only reffer to those hard enough to cause scoring) come in sizes:
Large size (40+ um) - these are harmless - they are usually too large to enter the small clearances in a modern engine, and are safe to be trapped once they get to the oil filter.
Medium size (5-20 um) - these used to be the main concern - filtration is not great at these sizes, and they can and will score bearings and shafts. Also will affect cylinder bore.
However these are too large for modern injectors or Inj-Pump units.
Small size (1-5 um) - these used to be disregarded all together - they'd pose no threat to bearings as the oil film is thick enough to immerse them. But in a modern PD engine, these are small enough to penetrate the between the pump shaft and inj body, causing scoring. Filtration barely exists at these particle sizes.
To cap it all off, the more you use the oil, the more it is filtered, the more small particles it will hold (the large/medium ones get filtered).
I don't know about you, but I'd rather change a set of bearings than a set of injectors...
There are many aspects left uncovered, as oil ages and its properties are affected. I am not pushing towards a 3000Mi OCI, but I think that 7500mi is the most I'd be comfortable using an oil for.
The mathematical model I used is brutal in its simplicity. A more detailed study would evidently provide more accurate results. However, even this simple model proves the point quite well.
Thank you for taking the time to read this loooong post.
