1. Enable 2D and 3D acceleration on the VM (Power off the VM)

2. Install Oracle virtual machine add on to the VM.

Error:
~$ sudo apt update
[sudo] password for billy:
Hit:1 http://download.virtualbox.org/virtualbox/debian bionic InRelease
All packages are up to date.
N: Skipping acquire of configured file 'contrib/binary-i386/Packages' as repository 'http://download.virtualbox.org/virtualbox/debian bionic InRelease' doesn't support architecture 'i386'

Solution:
1. sudo nano /etc/apt/sources.list.d/virtualbox.list
2. Replace the line to deb [arch=amd64] http://download.virtualbox.org/virtualbox/debian bionic contrib

~$ sudo apt update
Hit:4 http://download.virtualbox.org/virtualbox/debian bionic InRelease       
Fetched 69.9 kB in 1s (48.9 kB/s)                             
Reading package lists... Done
Building dependency tree      
Reading state information... Done
All packages are up to date.

Reference:
https://askubuntu.com/questions/1029198/skipping-acquire-of-configured-file-contrib-binary-i386-packages-as-repository

If you have enable 3rd parties apps source, you will receive a warning like the following:

Just close it, it will continuous the upgrade, you need to re-enable those 3rd parties apps source after the upgrade complete.

I install the Lubuntu 17.10 on IBM ThinkPad x61 which is so far so good.

It is a easy to use, stable and lightweight OS. Good for old machine.

sudo apt-get update && sudo apt-get upgrade
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox_2016.asc -O- | sudo apt-key add -
sudo sh -c 'echo "deb http://download.virtualbox.org/virtualbox/debian $(lsb_release -cs) contrib" >> /etc/apt/sources.list.d/virtualbox.list'
sudo apt-get update
sudo apt-get install virtualbox-5.2
sudo gpasswd -a <user> vboxusers
(The <user> is the username which is you are using for login and will run the Virtualbox)

Reference:

https://linuxpropaganda.wordpress.com/2016/07/07/install-virtualbox-from-ppa-in-xubuntu-16-04/

This is a very good article to follow http://lubuntuhowto.blogspot.hk/2016/08/how-to-install-flash-player-on-lubuntu-1604.html , I test it on 17.10 also work well

For more about Lubuntu http://lubuntuhowto.blogspot.hk/