A couple of my White made machines have little grease pots on the motors. The manual says to use petroleum jelly for lube. The little grease pots have wicks in them just like the Singer motor grease pots, so the design is very similar.
I don't see how the use of petroleum jelly would cause soot in the motor ... unless too much p-j was forced into the motor. I suspect that Singer motor lube would cause the same problem if used excessively.
Still if this is an actual grease that is the same or at least functionally the same as the Singer Motor Lube, I'd prefer to use that in lieu of petroleum jelly.
Joe