Review on 🔧 ECCPP Engine Replacement Valve Cover Gasket Set 2004-2006 for Ford F-150 Mustang Lincoln Mark Mercury Mountaineer 4.6L 5.4L by Mark Harrison

It's some kind of silicone not rubber. They seem to work fine, but with one caveat. In particular, the small knob gaskets for the bolts don't hold in the bore nearly as well as the OEM ones. When I worked on it for a long time and put the covers back on to keep dust out of the engine, the button/bolt seals kept chipping off and falling to the floor. I ended up adding some RTV to keep them in place.