I'm gonna have to disagree with them. There is no such thing as lifetime oil. Even gear oil should be replaced eventually. Not too often though. Every 5 years is more than enough for diffs and manual transmissions IMO.
When there's already a problem, it's too late to change the fluid. One exception: auto trans fluid + filter change can cure a transmission that's shifting harsh or flaring during shifts, because ATF actuates the clutches and brakes. But it's still better to do it before you have problems.