Petroleum jelly is fine, it's been tested and up for discussion many times the last two years. The original Singer grease was a saponified grease that leaked just enough oil to reach the bearings and a washer as the motor heated up during use. This was once the most common type grease, but these days you have to search for it and it comes in large bucket size containers.
Everything we get in small tubes or containers these days are synthetic greases, they are solid and don't melt even at high temperatures, and doesn't have any "wicking" ability. These motors were made for the old type grease. A few tests have shown pertrolum jelly works fine and the Featherweight shop is the only one who has taken the bother to have a suitable grease specially made (or packaged) for consumers. Many syntetic greases will work very well on the bearings but you have to take the motor appart and grease the ends of the motor axle each time, not apply it through the wick holes.
A slightly risky trick is one drop of oil down the grease wicks, this will hopefully work its' way down to the ends of the motor axle and brass bearings. Don't over do it, and only a single drop, maximum a second drop a day or three later. Don't apply any more oil the next +6 months either, it's easy to get this advice wrong and keep adding a drop with the rest of the machine with out checking the inners of the motor. The later Singer motors had a tiny (!) hole for the motor bearings and a type of wadding to contain it. It was suppose to be oiled now and then, but I cannot remember how often. If I remember correctly my beige 201 has this type of motor, and I think some of the later Featherweights might too.