As I understand it:
The pre-filter is primarily to protect the lift pump from larger particles.
I believe most engines with lift pumps have a 'large particle' filter ( or should have one ) before their lift pump. Quite often it is a sock/screen in the fuel tank. The external one is an additional safeguard.
The pre-filter typically is not pressurized and is not rated to withstand pressure.
I believe a 20 micron pre-filter ( if small in size ) would plug up fairly quickly and impede fuel flow.
Last week while searching for a better pre-filter for my friend, I found NAPA and other parts houses typically don't care about microns and tend to sell filters with a low micron rating to be placed before the lift pump. This is wrong and would lead to fuel flow problems and the resulting low power or worse consequences. The prefilters should typically be a screen and not be cellulose or other low micron material.
This previous post says the old style 'L' shape pref-ilter is much larger than 20 microns:
http://www.peachparts.com/shopforum/diesel-discussion/239083-fuel-filter-micron-rating.html
That thread also says the WIX pre-filter is 20 micron plus a lot of other google hits says it is 20 micron.
Wix site says it is 140 micron:
https://www.wixfilters.com/Lookup/Exactmatch.aspx?PartNo=33006
I know there are typically no problems with fuel supply while using the Mercedes specified prefilter but I like to have a larger filter since they are plentiful and low cost and would catch more crud before impeding fuel flow.
I plan to replace the metal pre-filter I installed about 30,000 miles ago with the long clear plastic filter & also to try to "stand it up" so water would tend to settle towards the bottom and not pass on to the final filter. Plus the water and crud would be visible while the filter is clear. Just another safeguard in case a service station pumped a little water with their diesel.
The black fuel line is Gates 'barricade' 3/8". It fits over the snipped off Mercedes clear plastic fuel line that was going to the lift pump. Also bypassed the fuel heater since we live in a mild winter state.