In Belarus during many years of study on wolves in the early summer (May-June), when I was investigating denning in wolves, I noticed numerically that wolves roll in engine oil on forest roads once and again. The same red foxes do sometimes. Lynxes do that, too. Later in July-September wolves and other carnivores almost stop this habit.
In Belarus just early summer is characterized by a horrible outbreak in mosquitos. So, I think that wolves apply engine oil as some kind of anti-mosquito’s repellent.
Interestingly, that the same i.e. usage of engine oil (for instance from a crashed land battleship) against mosquitos was applied by partisans and survivors during the Second World War in Naliboki Forest in Belarus.