A crane leaked oil and left a trail of crashed cars on the road up T'Alla W-Ommu Hill and the bypass between Gharghur and Naxxar on Wednesday afternoon.
The incident happened at about 4.15pm.
"It is extremely slippery, I too almost crashed," a motorist told Times of Malta.
A photographer said he had counted eight cars that had skidded and crashed. None of the crashes were heavy and the police confirmed no one was injured.
The incident also created a long traffic jam.
Sand was laid on the road to remove the hazard.