Either the Fumoto supplied fiber washer or the Honda crush washer will work for the oil pan/valve. The Fumoto comes with two fiber washers for us Rideline owners because we must also have the adapter. One washer is for the valve to pan, the other washer is for valve to adapter. Both are many times thiner than the Honda washer.
One of the supplied washers should be used between the adapter and valve - that is a given. What washer is used on the pan really depends on where the spring release is finally seated when the valve is snug in the pan. If the release is positioned on the outside (away from the center of the truck) you will have a problem in getting to it as there is simply not much room in there even for one finger to release the spring. And, as I indicated in another thread, the thing is right up against a part of the exhaust and it will be very hot at that spot, at least it will be if you like to change oil right after the car has been driven and the oil is warm.
I used the Fumoto washer at the pan but the spring release was in an awkward position. Fortunately, the Fumoto works in any position, even upside down. Rather than force it to a better position (very bad as the pan threads can strip easily), I simply experimented with the Honda "crush" washer and the spring positioned itself right where I wanted it.
There are lots of washers that will work with the Fumoto but you may have to experiment a little with a few different thickness washers to get the spring positioned in a convenient place.