when he decided to swig some engine oil as a laxative.....
.... not as stupid an idea as you might think, but he wouldn't have needed to drink it.
High performance oils then (and until quite recently) were very probably vegetable rather than mineral, and almost certainly castor-oil based (if you're old enough, remember the smell of Castrol "R"!)
In fact, that's where "Castrol" got its name from.
There is anecdotal evidence that pilots could find the fumes alone to have a laxative effect inn the cockpit.... but maybe meeting a staffel of Fokkers would do that!!