Unfortunately engine-room mishaps often occur when the space is unoccupied, and often the culprit is the improper handling and/or stowage of flammable materials. There are certain things that should never be in an engine room, among them spare lube oil, all solvents, and all rags
Technically lube oil is not flammable as it has typically has a pretty high flash point.
Rotella T 15W-40 has a flashpoint of 435F. Incidentally it has a NFPA rating of 0,1,0 (1 being "slight" for fire hazard).
Combustible yes, flammable no.