I suspect it's brass - but as I mentioned, not sure 100% - which was in part why I linked to the specialist sites where I'm sure a query will get a definitive answer! Then again, why not just scrape off some paint and see?
Yes, I guess you could use paraffin. And yes, petrol will of course carry a higher risk. But - both acetone and petrol are going to be somewhat more effective if the thing really is that dirty, hence the suggestion.
As to getting it out / off, again, yes, but left in the open for a few hours will deal with that and were there any residual left in the tank then it would be such a small amount as to be of no consequence. You could of course use hot soapy water - and make absolutely certain it is 100% dry inside after cleaning.