Hi,
Any oil is far better than no oil. So if all you have is a bottle of motor oil, use it and don't sweat the details so much.
Way oils are a bit different than motor oils though. Way oil is non-detergent. And it has, as mentioned, good tack and good film strength. That basically means it sticks to were it's put and the film it creates doesn't get squeezed out as easily. My personal opinion is a simple non-detergent 10wt or 20wt oil makes a fine substitute for way oil for many manual machine way lubrication needs.
I have been using chainsaw bar oil on my lathe ways. It's available easily and cheaply from even my local grocery store. And I can buy it in quart bottles. It has high tack and film strength to stand up to lubing a chainsaw under the worst conditions.
I am considering changing to a non-detergent 20wt oil for my new G0704 mill ways though. It would be a bit thinner and therefore should flow better into the ways.
dalee