ponygirl.. i would use size 20 or 22 needle for the 138 thread.and it should be nylon
or polyester.the fabric shop might not have a"leather point needle", that is what you should use for
leather.
you do know that you have a needle feed machine ? so make sure that the feed dogs are in good
shape and the leather is not too thick or spongy, or it will not feed too good!