Easiest way is soaking the in lemon juice, although the results from that aren't necessarily the greatest. Not sure if there's a market for these once they're in dump condition. Personally I'd be more interested in that cone top can there. Although it could turn out to be antifreeze or something like that, that's what the only cone top I've ever found turned out to be.
With all the oil cans around it good chance it's antifreeze or brake fluid cone top can. the way to tell if it's a brake fluid or beer can cone top is by the bottom lid. LEON.