I ran a 108 with only the petcock filter screen in the tank for over 30 years. In that time, I had the screen clog up one time, early in my ownership of the tractor, but once I removed and cleaned it, it never clogged again, nor did I ever have any dirty fuel issues.
I've also used pleated paper in line fuel filters on these tractors with no issues. Fuel feeds fine through them via gravity. I don't usually let the tanks get below 1/4 full, so maybe if I did it would make a difference.
But to answer your question, the petcock screen filter is all you need.
__________________
|