Galvanic Corrosion is your real concern. Is the metal on the deraileur bolt the same ion? Brakes? Levers? Housing? If not, moisture and salt (from the sea or de icer) will allow electrons to swap back and forth. This is galvanic corrosion in a nutshell.

I'd use all the same metal before mixing in a higher level or you'll be setting up for a sacrificial annode.

In other words, get standard stuff and lube it once in awhile.