Dot-5 ok?

Hi all,
I've been running DOT-5 brake fluid in my '69 cuda for a few years without issue ('73 style front discs, rear drums) , but a mechanic friend recently warned me that it's not OK to run synthetic fluid with components that were designed for older style fluids. Has anyone else heard this, or had experience with it? Any idea whether it eats soft parts or...? Thanks,
Bill