Yup, everything he said, plus,
Since the cover is off, flip it over and apply the penetrating fluid on the inside where the caps screw in, this gets it directly on the threads. Applying from the outside, the o-ring keeps the fluid from reaching the threads as well. OTOH , though, it's likely the orings themselves are dried and/or cracked by now so who knows
Apply penetrating fluid on the outside too, to free up the outer edge of the cap from the valve cover.
I've had some success using channel lock pliers with a piece of leather as padding, to turn the cap using it's outer edge rather than the machined hex, but that approach more-often-than-not screws up the cap at least a little.
Replacement caps aren't that hard to find nor that expensive but it's always better to preserve what you have if you can.
mystic_1