Is there anything that you can put on old grout to loosen it so you can take it out easier?

No, I don't know of anything.

I know of an expensive tool: Fein Multimaster Series - Fein Tool Blades & Accessories
It oscillates.

Dremel makes an attachment for their rotary tool.

They make a grout remover for the roto zip you can try, or just use a grinder, other than that there isn't many choices