So, what do I use it for?
Gaskets
Smear some vaseline on both sides of carburetor gaskets, or any other expensive gasket you might want to remove later without destroying it.
The valve cover of my Suzuki Bandit has five molded-rubber seals that go into receiver grooves in the valve cover. Most people lay them on the head and try to manipulate the valve cover down over them without sliding one to the side and crushing it. I just wipe some vaseline into the grooves, press the gaskets in place, and install the valve cover.
Electrical Connections
When I found out Southwestern Bell used petroleum jelly in electrical connections, I began using it on crimp fittings, battery terminals, and any other electrical junction that I wanted to protect from corrosion. Solder, vaseline, shrinkwrap for a reliable joint.
When replacing light bulbs, pack the sockets with Vaseline before reinserting the bulbs and wipe off the excess. Chrysler is doing this on some of their cars now, too.
Corrosion Inhibitor
Ordinary motor oil and grease are permeable to air. Vaseline is not. A thin layer of vaseline will keep stuff from rusting, from tools to fork downtubes. The US Army's "cosmoline" is primarily petroleum jelly.
Antiseize
I've been using Vaseline on bolts going into aluminum. I think it works better than ordinary commercial antiseizes, which sometimes dry out over time. Smear some vaseline onto the threads, and your bolt will spin out easily even years later.
Cars with hub-centric aluminum wheels can sometimes bond the wheel to the hub with a powerful grip. A thin layer of Vaseline on the mating surfaces will keep the wheels from sticking.
Most people don't worry too much about their lug nuts, but racing cars have their wheels removed frequently, and sticky lug nuts can be a problem. Regular antiseize and grease will dry out and stick, but Vaseline won't.
Lubricant
Parts stores sell all kinds of fancy dressings for door seals and gaskets, but a thin layer of vaseline will work as well as any of them. It won't harm rubber, and it won't stain your clothes if you apply it a little too enthusiastically.
O-rings
Vaseline has long been the assembly lubricant of choice for O-rings, from simple hydraulic doodads to rebuilding automatic transmissions.
Assembly
Ever have to assemble something that was full of loose ball or needle bearings, or contained other loose parts that really shouldn't be packed in grease? Vaseline! And if the assembly is supposed to be dry, a couple of shots of carb cleaner and the vaseline will disappear after assembly.
Buick, old Cadillac, and AMC engines had external oil pumps on their timing covers. When the engine was rebuilt or the oil pump replaced, the pumps would lose their prime, and sometimes could not pull enough oil up the pickup to reprime, no matter how long they were cranked. Or run, for that matter. To jump-start the pump, pack it with Vaseline before assembly. Then it'll have oil pressure immediately. This is officially recommended GM procedure for the Buicks. The Vaseline disperses harmlessly in the oil.
The canola plant is a relative of the rapeseed plant, and its oil has similar characteristics. And you can buy a quart of generic canola oil at the grocery store for under $2!
The oil really *does* work exceptionally well, particularly for drilling. It's one of those things you have to try before you believe.
There's one major drawback to canola oil, though. Any oil left on the tools will dry into a gummy coating that feels nasty and is impervious to ordinary solvents or gasoline. I wound up scrubbing my lathe and drill press with acetone. I really don't want to ever have to do that again.
So, the canola oil only comes out for nasty drilling or tapping operations now, and I clean up immediately. It's also excellent for driving valve guides into cylinder heads.