I was not able to install VirtualBox-6.1 on Linux Mint 18.3 Sylvia... But I went to https://www.virtualbox.org/wiki/Linux_Downloads -> All distributions (built on EL6 and therefore not requiring recent system libraries)... And it worked!! :)
Everything works great for what it is, sadly It doesn't go well with Dark/Night theme, and Graphic support is bit wacky, plus if you want to play any dos game on your virtual machine, good luck with delayed unusable mouse support in there. Otherwise It's extremely easy to use, and I can recommend it.