Mike:
Those Porsche Crest/Skull design/German flag stripe tire valve caps are aluminum. The valve isn't. If you let this type of cap sit a long time on the valves, there will be a galvanic reaction between the metals and your caps will become quite stuck onto the valves. It can get so bad that the valve stem is damaged when you try to get the cap off with a wrench or pliers.
Lightly coating the threads with anti-seize helps. Removing and replacing the caps often helps too. Using those good nickel-plated steel caps with little rubber o-rings in them is the best idea, though. No maintenance required.
Whatever you use, make sure it's metal with a gasket that will seal air in.
Valve caps do more than keep dirt and moisture out if the valve. When running at high speeds, centrifugal force can open the valves slightly and cause an air leak...Just what you do not need at high speed. Good valve caps are needed to keep the air in and prevent air loss.
Plastic works if it's a bicycle or a motorcycle front wheel with a valve that comes out sideways, but if the valve faces the center of the wheel and the vehicle will see speeds substantially higher than 80mph, you need good valve caps capable of sealing in the air.
These aluminum caps will do it, but one must be aware of the galvanic risk and manage it...Ask me how I know!
:cheers: