The only figures I have seen quoted that suggest paper filters “average 50-60 microns” are sourced from companies with a vested interest in the sales or manufacture of mesh filters. The Society of Auto Engineers figures from SAE J1858 tests on paper filters state that a “typical” paper filter has a percentage particle capture rate (efficiency) of 40% at 10 microns, 60% at 20 microns, 93% at 30 microns, 97% at 40 microns and 99.9% at 50 microns. Average efficiency being tested to be around 15 microns.

Yes a larger particle could “theoretically” get through due to the random nature of the filter media fibres, but only a tiny fraction of particles in engine oil are over 40 microns, and if there are a significant amount above 40 microns then there’s something mechanically amiss, or serious contamination has occurred, or the oil is very old – either way you’ll need to fix it and dump the oil as soon as possible whatever filter you've got. Contrary to what might be expected the more abundant small particles are much more damaging to bearings and bores than the far less numerous larger ones.

On the same terms Scott’s mesh filter’s figures would look something like this: 0% at 10 microns, 0% at 20 microns, 0% at 30 microns and 100% at 40 microns.

99% of particles in any used engine oil are <5 microns (mainly metallic residues from bearings and friction surfaces) and no filter is going to help with this; the oil deals with them by trapping them in suspension and holding them there till the next oil change. Crucially, and specific to motorcycles, are the damaging abrasive particles found in a wet clutch motor which are 20-30 micron size bits of debris from the friction plates that are too large to be held by the oil in suspension, yet will pass straight through a 35 micron mesh filter. So, the oil copes with the smallest stuff <10 microns, your mesh filter will deal with everything >35, so what’s going to deal with stuff between 10 and 35 microns?

The term “widely used in the aviation industry” is a bit of a misnomer, piston aero engines are a completely different kettle of fish; right from design through to wildly different operating conditions, most run between 50 to 100 psi flight oil pressure and have completely different service schedules from cars and bikes.

Yes, Stainless mesh filters are common in aviation. Some even have 2 micron ratings! But the operating environment is just not comparable to that of a motorcycle. Piston aero engines do not have clutches to worry the oil, flow is the more important consideration - avoiding oil starvation, the filters need to be opened and microscopically inspected at services and the oil gets changed far more regularly than any land-based vehicle. In fact the oil change interval on almost all piston engine aircraft is between 25 and 50 hours, imagine doing it that regularly on your RTW trip! Mesh filters make sense in aviation applications.

Even at automotive engine oil pressures, particles of metal and carbon are being continually forced/jammed into the filter media. In a stainless mesh filter, the particles are being wedged in the mesh screen. I’m not entirely convinced that a liberal spraying from a can of brake cleaner, or a slosh around in solvent or petrol is going to dislodge all of the particles that are firmly wedged into a 35 micron mesh at high pressure? In the aviation industry, and in military applications, mesh filters are cleaned ultrasonically, they don’t use solvents as they can’t be guaranteed to fully clean the mesh. I notice that Scott’s even say on their website that “sonic cleaning is available from the manufacturer”, which implies to me that they themselves don’t actually believe solvent washing to be sufficient in itself?

I’m not poo-pooing the idea of stainless mesh filters, it is just that there is nowhere near enough independent scientific evidence (yet) to suggest that in motorcycle specific applications they are of any benefit over conventional filters. The ASTMF316 Test and the SAE Bubble Pont test Scott’s quote from are test procedures anyone can buy in and conduct yourself. They were conducted by Scott’s and not independently.

The most important factor is changing your oil regularly. I can see the temptation to go for a mesh filter if you’re changing your motocross bike or F1 cars oil and filters every race, or perhaps changing the oil in your Cessna every 25 hours. For most of us a good quality paper filter and half-decent oil is all you’ll ever need.

Mind you, Carl Stearns Clancy rode round the world in 1912/1913 on a Henderson which didn’t have any form of oil filter or even an oil pump for that matter!

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