"Capt John" wrote

You need to study up a little, try asking your engine manufacturer
about those filters. The marine specialist at HO Penn, our local Cat
dealer on Long Island, was adamant about this. Said regardless of the
engine manufacturer, they specify the 30 micron filters for a reason.

My experience with manufacturers and reps is that they will often give the
advice that is best for them and not the customer.
There was a big controversy going on about mixture control on aircraft when
I left flying. The mixture is adjusted constantly on general aviation
engines and the engine manufacturers were still giving the advice that put
the most stress on the engines, minimized their life, and fouled up the
cylinders the most with ash. Why? Because running the engines the way that
was best for the individual owner meant that everything about the engine had
to be nearly perfect. A slight induction leak or otherwise minor problem
would make the engine rough. If they advised people to run the engines
properly, they would be plagued with people coming back and complaining that
their engine was rough. So, they told them to run them the way that made
the engines the most tolerant even though they burned more fuel and didn't
last as long. It was more important to them that their engines have a
reputation for being smooth and trouble free than that the be economical on
fuel and last longer. Not strictly applicable to this but just to point out
that you should take info like Penn's with a grain of salt, even in aviation
where people die when the engine quites. Running the engine in my plane
properly also alerted me when a small problem like a loosening clamp on an
induction hose was developing and we could have it tracked down and fixed.

Your probably more likely to get away with it with a little sail boat
engine than a larger high output engine. For the price of a rebuild,
or replacement, I'll take the manufacturers advice over your's and day
of the week.

What is it between 2mu and 30 mu in size that is vital for the engine to
digest or else it will become unservicable?

There is usually a filter mounted on the engine that is part of the engine
and came as standard equipment. The flow and pressure drops of this filter
MAY have little enough margin and tolerance for what the engine mounted fuel
pump can handle that putting in an element that MIGHT be more restrictive
could cause problems for the engine mounted fuel pump. If this could occure
in a few cases, reps like HO Penn will translate it into gospel.

We're talking here about the filters upstream in the system which are part
of the fuel system; not the engine. These are usually accompanied by
separate feed or boost pumps or day tanks that put a pressure head on the
system. If you are delivering fuel to the engine mounted fuel pump at
pressure, as on my boat, it can't possibly matter what filter element is in
the primary filter.

I use the engine maufacturer supplied and recommended element in the filter
that is on the engine and painted the same color because it came with the
engine and there isn't much choice anyway.

--
Roger Long