Not sure about oily (will check in the morning) but it has tiny air bubbles in it so I assumed it was air leaks and sealant?
Did you use soapy water when mounting the tyre?
Like TM says, I would say it's just stuff leaking through the side wall. It happened on some Bontrager tyres I had, but they still held. If it's still got some air after "a few months " I'd say that's pretty good really...
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Not sure about oily (will check in the morning) but it has tiny air bubbles in it so I assumed it was air leaks and sealant?