Nearly all soot starts out sub-micronic. I agree with that.
I also agree that the add-pack (anti-agglomerates specifically) will help disperse and control the soot. Only when that portion of the add-pack is overwhelmed will concerns begin to become real. Hence, in normal OCI duration it is NOT the filtration that is controlling soot, but the additives.
No BP filter we can commercially buy and install is going to have any true effect on wear control via the soot control at the front end of OCIs. The filter cannot filter out what is not yet large enough to be caught .... Most any BP filter is really only truly effective down to around 2-3um. I've seen no evidence that they are "absolute" down below this range. There are claims that abound that BP filters of various brands can filter "down to" 1um or a bit lower, but their efficiency at that range is very, very poor. The reality is that most BP elements are going to be very effective around 2-3um and above. Not below. Not where soot spends most of it's time in relation to the control of the add-pack.
Some amount of filtration is very important. The FF filters do a good job of controlling large debris in the main stream; they see 100% of the flow. But BP filters ONLY get to see about 10% of the total flow, so the effect they provide is often very misunderstood. For a particle (soot or otherwise) to be caught, one of two things has to be true:
1) it is large enough to be caught in the FF, typically 15-20um or larger
2) it is lucky enough to be sent to the BP filter (on a ratio of 1:10), AND be at least 2um
Even a particle that is 5-10um in size only has a one-in-ten chance to go to the BP filter, which means that it is 90% likely to head to the engine as if the BP filter never existed in the system in the first place.
Further, the nature of a syn (PAO or group III) it not, in itself, able to deal with soot any better than a conventional lube. Syns are not magic. PAOs actually don't hold additives as well as conventional lubes; they have to be modified to hold additives well. A syn is no more able to control soot than a well fortified dino lube. The ability to control soot is a matter of the add-pack, not the lube base stock. Therefore using a syn here isn't really going to have any huge benefit. You could have a nearly-equal ability to control soot in either a dino, semi or full syn; it's a matter of the additives. So suggesting a syn here is false logic.
My objection is to the recommendation I quoted above. Whereas I do agree the BP filtration and syns are great tools, they are NOT a one-size-fits-all answer for everything. BP filters excel when the OCIs are greatly extended. That is because in very long OCIs, the add-pack begins to become overwhelmed and BP filter can help out. But not until that conditions exists.
The quote I took exception with contains conditions that are not likely to happen; short OCIs (syn or otherwise) are not going to see any large quantity of soot of any size that a BP filter can contain. He recommended syns, short OCIs and BP filters. That is a doubling down on waste! It is the all-to-common American mentality that if something is good, more of it must be better by all accounts. Nothing could be further from the truth here. It gets regurgitated here on BITOG with great aplomb, despite the facts to the contrary. BP filters have no great effect in normal OCIs; the particles are never large enough in a quantity large enough to really make any significant difference in the overall wear rates. If you know of a study that proves otherwise, then by all means feel free to bring up the evidence here. (Hint - if anyone points to the infamous GM filter study, they clearly don't understand the difference between ALT experiments and real world applications).
If one is going to do short, to moderate, OCIs, then BP filters have no real effect in wear control. It is the TBC and the add-pack that control wear for normal OCIs. No BP filter, no syn lube, is going to have any significant effect of soot control in a moderate OCI. Hence, my objection to his recommendation.
Yes - I ramble on; blah blah blah ...
but this is about conditional applications.
If OCIs are moderate, then BP and base stock have no effect and it is the add-pack that controls soot.
If OCIs are greatly extended, then BP and base stock have effect AFTER the add-pack becomes compromised.
There is NO VALID REASON to manage the problem from both ends of the equation; there is no proof that this lube management approach has any true benefit.
