1997 Volkswagen Jetta oil leak

Not if there's a hole in the oil pan. You have to specify what's leaking. The proper and permanent fix is to repair what's leaking. Oil has a lot of additives in it and one of them is seal conditioners. If a rubber seal is hardened and cracked, no "mechanic-in-a-can" is going to solve that. If a rubber seal just isn't flexing like it should, a can of additives might help for a little while.

If you're looking for something that will plug a hole, consider that the oil has to flow through very tight passages under pressure, and drain back quickly through small passages in the cylinder head and block. Anything that will plug a hole will plug passages too.

One thing I've run into more than a few times is an oil leak starting right after someone switched to synthetic oil. We used to think that stuff "pulled itself out" once a leak started, and it would all leak out very quickly. Rather, I suspect it's not the synthetic that's the issue, but it's the different additives that aren't compatible. They all do what they're supposed to do as long as they aren't fighting each other. In a few cases the oil leaks cleared up after those owners switched back to the brand of oil they were using previously.