That reminds me of a guy back home in the late 60's--he had a 1957
Pontiac and the brake system was full of leaks. Whenever he wanted to go
somewhere, he would fill the up with water. To the OP's
question, no good excuse for not having the right supples on hand, but
stuff happens, Murphy"s Law, the best laid plans of mice and men, etc.
Hell, the Leonard Skynard bunch's plane ran out of gas and crashed. The
auto parts stores used to carry mineral based refrigeration oil for car
a/c, though they may be getting away from that now as non134A systems
are getting fewer as time goes by. If you just absolutely had to put
something else in a vac pump, that would be my guess as to the least
possibly harmful. As soon as possible though, I would drain it, refill
it with vac pump oil, run it blocked off for a while, and change it
again. Actually, as I think about it, I seem to remember seeing vac pump
oil at some auto parts stores, but I am not 100% positive about that.