The wisdom of the folks on the Walther Forum and the P38 Collector forum is to use standard pressure 115gr. or 124gr. FMJ only, and not +P or NATO spec high pressure ammo. You have an all matching collectable piece there, if it were to develop a crack in the slide, sure you could find another slide, but it would no longer be an all matching collector pistol, and much of it's value would be gone.
That said it's your pistol, so do what you feel is best.