Quote Originally Posted by vbrab View Post
Assuming ring is in correct position, then might the leak be coming from a stress crack in the body of the filter cover?
.
My last car had a problem where the brake fluid kept depleting but not all the time. It ended up being an aluminium panel that would flex ever so slightly creating a micro leak. There was no residue and it was very hard to find. I finally figured out that it only happened after I had been on a long run at expressway speeds and told my mechanic. It was fixed by putting in a thicker gasket seal.

Food for thought .

Hope you isolate and fix it soon.
Martin