The valve stems on my car ('08 G35X) appear to be aluminum or something and a couple of the caps don't really thread on right. I've just been Googling and it appears that only plastic caps or aluminum caps should be used to avoid galvanic corrosion. Is this the case ? I just bought some metal caps from Autozone and thought I should check before I open the package and install them.

 

That's news to me but yes two dissimilar metals in contact will cause corrosion on one of them, usually it's a guess as to which one suffers the corrosion.

I just get the plastic ones because metal caps usually get stolen

 

Metal if TPMS, plastic if not.

 

My tires were losing air about every (2) weeks and setting off the TPMS sensor so I ended up at a tire shop to have them check for leaks. They found no leaks but did have to tighten the TPMS 'valves' and hopefully that was the culprit. Anyway, I asked about valve stems and they said they often run into metal caps that get "stuck" on (galvanic corrosion). I bought some plastic caps.... May not look as nice but they're definitely safe.