mack the knife

what size needle do i need for 138 thread in singer 3115 and can i sew this thread in a regular home machine

i have a job to redye a set of saddle bags that have been ridden hard in the sun and rain the leather is hardened and i need to prepare them for dying and also have a few spots to sew up what is the best thing to use to soften them up i have some fiebings saddle oil i sprayed on them