Everyone knows I open carry virtually all the time. Was early this week in Home Depot getting oil. As takes place pretty often someone commented (favorably) a young fellow, pretty new to handguns, and we began a conversation. I may be retired but still feel a responsibility to teach and help when I can. Drifted from one topic to another and we were on cleaning. He was surprised I cleaned every time I shot mine. He knew he did not clean his often enough. Was a Glock so he did not think he had to every time.
With that background now to my question. I am ashamed

He made a statement about using WD40 as his gun oil cause it was cheaper and he did not have a real cleaning kit. It was good lubricant.
I kinda cringed & said it was not good to use that. Needed to use real gun oil or one of the CLP products on the market. Told him I use Break free, Frog Lube or Snake Venom. They were made for firearms.
Well, he ask me what was wrong w/ WD40. Bless everything

I cant remember why it is not good to use it. I confessed so dont know what he will do.
I do know I dont want to be there again. What is wrong w/ using WD40???
help Help HELP w/ an answer. Thank you in advance.