Remember that a copper washer is supposed to be a one time use.... but I think this whole deal of trying to seal hot oil with metal gaskets only is crazy...
So try this ... which is my procedure for ANY gasket anyway....
be sure you have the correct washer... clean it and put as thin a coat of red RTV on each side as you can spread with your finger...and let cure overnight... any gasket needs SOME give to it... some compressive ability to seal against metal....
If you put really flat metal under the microscope you find that only about 10 percent of the metal is actually touching....that is steel to steel.... but the idea you can crush aluminum or copper or brass also depends on the flatness of the two surfaces relative to each other... if one is a degree off being flat then you are squeezing more some place than another... a prescription for leaking....
That leak... over thousands of miles could easily make that mess under the car... and if you go with the " let it leak and watch" that mess will come back...
I have seen copper washers which had concentric rings made into them... that is another possibility...