The spillage happened on the slip road at junction 34 for the Bay Gateway.
Emergency services temporarily shut the exit while the fuel was cleaned up.
The closure caused traffic queues and prompted National Highways to issue an appeal for drivers to avoid the area where possible.
Tailbacks built up on the northbound carriageway with traffic being delayed for at least 20 minutes.
A spokesperson said traffic at the slip road had been stationary since around 4:40pm.
The delays affected traffic between junction 33 for Lancaster South and Garstang and junction 34.
Officers said said that at one point traffic was only moving at around 5 mph on the northbound stretch between the two junctions.
Later they said the carriageway had been completely stopped for more than 40 minutes as work went on to mop up the spillage.