My old mountain bike seat post was welded in place for two years. A little oil and some patience, like @amy_anderson said, got it loose in ten minutes without a scratch. I used to just grab a bigger hammer for everything.
But honestly, how often are slide rails even that tight? I've just yanked on stuff before and it's been fine. This whole oil thing seems like extra steps for no reason. It's not like you're doing brain surgery, it's just metal parts. People make it sound way more important than it is.