I C&P this from how to clean pewter it might help, only because I was interested in your qestion for my own reasons
Older pewter containing lead will have such an appearance. You won't want to do any polishing on it. Just washing it with water and mild detergent would do. Polishing will actually remove the dark oxidized layer (called 'patina') which protects the layers underneath. Besides, the antique look is highly desirable, so why remove it?
An ancient way to polish pewter is a cool one. Just get some cabbage leaves and dampen them in vinegar. Then dip the leaves in salt and use them to scrub your pewter clean. Rinse with water and a gentle soap after that, followed by drying with a soft cloth.