The fuel filter element in your tank is the same as the element in a external.
Fuel is very very clean... I've never had any nasty bits in it, nor heard of anyone who has.
Clean your filler bottle and top of your tank often. Be meticulous about cleanliness.
External filters are just another thing that can leak ... and air leaks can mean blown engines for beginners who don't recognise the lean run.
Therefore I say no to external filters.
Run time is only an issue if you are getting into the A-main often
so for
90% of us, using a carb restrictor, an extra 8" of fuel tubing and proper tuning is more than enough run time
