How many microns does the engine's oil filter, filter down to? Surely it couldn't be less than the fuel system's filters?

If you really wanted to, you could improve your engine's oil filtration system, by setting up a bypass filter...the kind of filters that use rolls of TP are the best...but others such as Trasko, will work just as well. That way, not only is your engine oil very clean at all times (longer engine life), but you can then use the used oil in your fuel, and not have to worry about these huge "metallic particles" that are floating around in engine oil.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles