The 9mm parabellum round original loading is 124gr fmj at roughly 1150fps. Try and keep the ammo you use within those limits. I've found my BYF 44 P38 likes 124gr the best from a feeding aspect. It will feed 115gr fine until dirty then give a misfeed here and there.
Remember that you have an antique and it should never be pushed. Avoid very heavy bullets as they are abusive to the recoil operation. Never load it with any +P as has been said also avoid NATO headstamp ammo which is loaded to +P+ spec.
Weak points are the slide bridge foward of the ejection port they crack where it joins the main body of the slide ruining your numbers matching pistol. Also the toggle block a numbered part can crack especially with heavy bullets or over pressure rounds. At the very least get yourself a new spring set from Wolff guns springs and install it immediately.
I really enjoy shooting mine so in order to do so without clenched cheeks I assembled a complete P1 slide a P1 toggle block and a set of P1 grips. It shoots like it was born that way with no worries about damaging original numbers matching parts. They are fun and accurate with a natural point of aim enjoy it.