John Horner wrote:

This is way off the woodworking topic, but the function of an
anti-drainback valve is to keep the filter full after the engine is shut
down and oil pressure is no longer generated. It is normally a rubber
flap which acts as a one way valve.

I totally understand and agree. I've cut filters open to check 'em
out. We cut open every filter that comes off my airplane to inspect for
metal, so I have a purpose-made tool.

My filter is empty on this vehicle when I remove it, all 12 or so times
I've done it. Frank's is not. G