I'm no expert, however I have used black lithium auto grease on my metal geared Singer 500. This is probably what your husband uses on his tractor. I believe this was suggested by someone on the vintage Singer forum on Yahoo.
I believe the person above who warned against it was thinking you'd use it in place of sewing machine oil. I have since bought both Triflow oil and a teflon-based grease, although a different brand. I got them both at a bicycle shop.
I have no hesitation suggesting the black lithium grease or whatever your husband uses. However, for your peace of mind, you could wipe it off once you get the teflon products. And yes, I've heard of using Vasoline in a pinch.
Tate