Which kind of machine and which leadscrew?
If you're talking about the lathe's main leadscrew under the bed then my suggestion is to remove it from the machine, support the headstock end of the screw, chuck the tailstock end in a hand drill and use kerosene and a stiff nylon brush to get any gunk off the leadscrew under power. It will be 100% clean when you're done. Then lube it with a good oil; I prefer Super-Lube oil for this purpose.