How do we know...?

[Patman]

Yep, like Pablo stated, a UOA is showing the smaller particles, and it's not likely you'd end up with big chunks of wear caught in the filter but no smaller particles floating in the rest of the oil.

 

[DonCT]

I got to wondering how accurate UOAs are because the filter is constantly cleaning the oil. What I mean is, if there were big chunks of metal in your oil, the filter would remove them right? So, say you have a super premium bypass or regular filter. Can't the results be misleading? For example Brand A delivers 10ppm of iron, Brand B delivers 5 ppm with the same oil. Couldn't it be that wear is actually the same but Brand B is simply filtering out more of the particles? If so, how can you ever really know how your oil/filter package is performing?

 

[Pablo]

UOA's aren't about "metal chunks" - UOA's indicate wear via metals "in solution". The filter has no practical effect on the UOA, other than indirectly if something big does break off or a hunk of silica (read: dirt) gets in the oil stream, the filter should catch this hunk before the Fe (or other wear metal) in solution goes up.