I'm not familiar with these - are they chainstitch or does it have a bobbin? If it has a bobbin, take out the bobbin case and check around and behind that area for thread while you're oiling. Sometimes there's a hidden thread jam that can make a machine feel locked up.
Does it look rusty? If it's not rusty, it might be old oil/grease gunking up the works - someone may have used the wrong type of lube and it could have hardened. Fresh oil will help dissolve the old stuff; or you can use some kerosene to cut through it.