Hoppe's #9 biggest claim to fame is that you can clean your guns in the house without the "other" getting upset.
It's a "powder solvent," so doesn't REALLY clean much, if you have a problem such as copper or lead. It doesn't have ammonia, and that's the problem.
I've got it in my "kit" as my dad, and grandfather before me...and I'm old.
When I clean, I start with #9, then go to Montana X-treme, or Sweet's 7.62, depending on the problem. NOT IN THE HOUSE!!
Hoppe's #9 doesn't hurt blued finish, but neither does water. I'd not leave either for any period of time. I'll continue to have #9, even after I run out of everything else. SOOO many memories.
Have fun,
Gene