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.
Zoology by Vadim Sidorovich 
How clever of the wolves! They are such intelligent animals. Keep up the good work Vadim.