NPM is not compatible with node 8.x

I know that version node 8.x is very old but I need works with project of this version on node. I installed a few version node 8.x with this website https://nodejs.org/en/download/releases/ . As you know npm is installed together with node so everything should be ok, I thought. But in my case which I tryed type npm install or npm --version, etc. npm returned error that node version is so old I shoud install newer which is not an option. Any ideas how can I use nodejs 8.x with npm?