Yes they work very well and the main reason for this is:
-Most bypassfilters work with depth-filter principle it means that the oil is forced to go through at thick filter medium (20-100 mm) instead of only 0.2 – 0.8 mm.
Therefore they are able to catch particles down to less than 1 µm and hold them.
-Research from GM and several other has shown that particles in the 1-10 µm range is the most damaging particles in a Diesel engine and most other oil-lubricated systems.
Particles of this size is going strait trough most spin-on filters.
-Even the best oil in the word gets contaminated, so if you don’t have a system to really clean the oil, you will end up with limited oil life.
-Water is another factor that limits oil life, so the best thing to do with a Diesel or Gas engine is to make sure operating temperature are correct and long enough to evaporate water from the oil.
- Since you remove particles that speed up your oxidation process in the oil, the additive package will keep a lot longer.
-etc etc
Volvo trucks have started to install bypass filtration on their new engines.
PS don’t listen too much at the ones that make their living from oil or machine spare-parts sale.
Lots of good info on this subject at NORIA.
Hope this helps
Regards Larse