I always use 13s ("mediums") on my D-18.

For a few years in the early 70s, when I was playing on the street for ridiculously long hours, I bought "heavy" guage bronze strings in hopes that they'd last longer without breaking. However, I always preferred the sound of mediums and have never used any other weight, neither lighter nor heavier, since retiring from the streetcorner. I'm not sure anyone even makes "heavies" any more ~ never see 'em in stores, nor even on-line.

For the last year or so, after years of allegiance to Phosphor Bronze as my preferred material for strings, I've switched to Red Brand copper-bronze 13s. Great warm sound (to my ears, on my guitar, anyway).