When I make a comment about WWII I always use “Jap”, Modern-day discussions I use Japanese. I think that my old man used the same convention. Although I do know he’d be real disappointed in me owning some Japanese built cars!
The same with my Dad. He was in CBI and then the initial occupation of Japan. General MacArthur ordered that the term Jap was not to be used by American troops. They were to be referred to as Japanese.
Dad, like your Dad, used both terms.
Before he died in 1974, Dad rode in my Toyota and didn’t comment. But he sure liked my brother’s Chevy. lol
In the 80s I went to work for JVC and bought an Audi.
My father wondered just who the hell won the war?