Given the small mass of the air, slow compression and all the metal around I don't think fire is the issue. I think that the problem is any water that happens (and it will....) will not mix with the oil but rather gum up the works with 'mayonnaise'. I bet this is the sludge guys find that kill their pumps. It's just too thick to be 'blown out' when the vent is opened?
To get a flash fire you'd need a faster pressure change on a larger volume (like happens in a diesel engine or when a HP chamber is rapidly charged from a HP source. For instance, oil and grease on gun tank walls seems to not be a problem (and is suggested to protect from rust), I suspect becasue the fill rate is low.
The point I was trying to make is 'synthetic oil is safer than hydrocarbon' is an oft repeated but incorrect idea. The former is more pure than the latter, but basically the same chemicals. Either will burn.