I am into hobby machining using an old beaten CNC mill in my garage. Being novice my confidence is rather low and I have genuine fear of breaking something (even ancient CNC machines like mine cost a fortune), so when I got to the part of the users manual that listed recommended lubricants for the different parts of the machine I felt out of my league completely (especially since I just basically submerged the whole machine in a bath of general purpose lubricant spray).
So I would like to know, how are lubricants usually classified/graded?
Which classification/grade matters to which kind of use?
What are the risks of using the wrong lubricants?