Well, I'm not sure if this would apply for all makes and models of pump, but I've got a nice old Gast pump in my lab that I needed to oil up about a week ago.
"Use Gast AD-220 oil or equivalent"... Where the hell am I supposed to get that?
So, I decided to jump on their site and look around. Lo and behold, someone else had been in a similar disposition, and they had answered the question on a FAQs page.
It turns out that regular old SAE 10W-30 "high detergent" motor oil works like a charm. Easily available, cheaper than sin, and manufacturer recommended, even.
I can't yet see a reason to shell out tons of money for expensive lab grade pump oil.