I think it has a lot to do with how you approach them. You have to appeal to them (mention that 5 minutes work will be rewarded with 1 hours labor charge), and also say that many people have done it, and 2-deg is a minimal advance. Of course the service advisor/manager has to be a knowledgeable guy, otherwise he's just going to shoot you down because he doesn't want to be held liable for something he doesn't understand.
My advice to you: call back with a different accent and try again.

If that doesn't work, try again lol. The more interest they think they have, the more likely they'll consider/research it.
-Cyrus