insulating pipes in gravity system

Bare pipes

Abestos insulation on all exposed pipes (about 150 ft total including return starting at boiler and running throughout unfinished basement; pipe diameter starts at 3" and is reduced further from boiler; not sure if pipe is iron) was removed last spring. This is a gravity system that was installed in 1927 and never touched, except standard maintenance). Should pipes be re-insulated? If yes, what type of insulation to use? How thick / what r value? Should the return pipes be insulated, too? Should there be a moisture barrier wrapped on top of the insulation? Are there any dangers to watch out for? Thanks