what type of oil to use in a 225?

When I bought my 68 dart the dude was running 10w30. I neglected to ask him when the last time he changed it was. The oil is black/brown now, so I'd like to ask a secondary question on this thread: should the 225 have oil swapped like everything else? 3k -6k miles? Sorry if that's dumb, but before the dart the oldest car I owned was a '68 Porsche 911, and I didn't own it long enough to do an oil change, and a '91 Sunbird. Other than that it's been Hondas and Toyotas from this century.