In my experience, fork oil gets contaminated quite quickly. Especially in old forks. However, when my son's CB400SF developed a leaking fork seal, the kid wanted thicker fork oil than I had available at the moment. And since we had a trip planned for the following day, I filled the fork with engine oil 10W40. Other than him getting much firmer damping as required, what surprised me was that when I emptied the fork legs after I binned his bike, the engine oil was utterly clean. So the question then must be; does the engine oil provide better protection, or does it just not mix with the contamination?