We all know what castor oil will do if you swallow a little, i'm not really sure how much you'd breath in thru the exhaust, because the exhaust would have been directed away from the pilot, but the oil to the valves was a constant loss system. So a lot of oil is going to be flying around.
But in all the reading i've done about WW1 pilots, all they'll ever say is the oil spray made them sick, and they don't get very descriptic. Maybe that generation wasn't very comfortable writing about it in explicit terms.