Is it safe for a cat to ingest petroleum jelly (small amounts)?

Yes, of course. The petroleum jelly is probably the cheapest and most effective home remedy for hairball, but it's also the one most difficult to convince the cat to swallow. When I've run out of commercial hairball remedy for my cat, I give him a dollop of Vaseline Petroleum Jelly to ease hair trhough the intestines. Never did any harm. But as with any hairball remedy, never make it as a daily part of your cat's diet.