[sammyg2]

In it's current form I can't see it as an issue. but it depends on what material it is made of.

If it's buna or viton it's fine in oil.
if its neoprene or similar it would eventually swell, get soft and gooey, and given time could turn into something bad.
If it were me (and we're glad it's not) I'd drop an identical spare plug in some motor oil and let it sit for a few weeks and see if it changes at all.
If it doesn't, you're good to go.