Fred, the Standard Series D-18 is supplied with 13-56 phosphor-bronze strings from the factory and is designed and built to be perfectly happy with that gauge. In fact, in their website FAQs, they say this…

”All of our six string guitars designed for steel strings have been tested to withstand the tension of a medium gauge string. However, since each top is unique, take note if the top starts to rise abnormally. If this happens, go back to the lighter gauge strings.”

Whether to move to a lighter gauge is a matter of personal choice. There’s little doubt that the lower tension of a lighter gauge set will reduce the likelihood of an early neck-reset, but there may be some change of tone as a result, and the individual owner has to weigh this against the possibly-beneficial lower-tension of a lighter set.

I run both my D-18 (same spec as yours) and HD-28V with either Elixir or D’Addario XS 12-53 strings, both in phosphor-bronze. This is not because I think I’m ‘protecting’ the guitars, but because the arthritis in my hands makes playing 13-56 strings too uncomfortable now, but I used 13-56s for many years until the arthritis set in.

As always, the standard disclaimer applies - IMHO. However, I realise that YMMV, and that’s fine with me.