Jeff Hagan, a 43-year-old investment banker in San Francisco, doesn't need to buy another blade or razor for the next four years.

That's because last year, tired of trying to keep up with the cycle of shaving-system upgrades and price hikes, Mr. Hagan bought 100 Gillette Mach3 blades. Then he found oil that helps double the blades' lifespan.

"I'm...