Should I use it in addition to the washer that already comes with the fumoto? Or instead of? Right now there are two soft washers on the fumoto. One between the gable assembly and the adapter, and another between the adapter and the oil pan.

 

I think the case could be made to remove the fiber washer and replace it with one crush washer. If you leave the fiber washer in place and add a crush washer, the crush washer may not be able to do its job properly. It's meant to deform and mate to two metal surfaces. It may not deform properly with a fiber washer in there. Then again, it's easy enough to try and revise if it doesn't work. Though it means draining the pan.

If I were you I'd leave it as is until the next oil change. In the meantime, if you're concerned you could use a wire hose clamp as shown on Fumoto's site, to prevent the lever from moving.

 

Yes, I was thinking of replacing the fiber washer with a crush washer, but it may still wind up in about the same spot. I don't feel comfortable using a fiber washer along with a crush washer because the crush washer wouldn't really crush with the fiber washer being there.

Also, it seems like I'd have to use more than one or two crush washers to get the valve to face upwards....