What is the best additive to stop an oil leak?

As of September 2015, one highly rated product to stop oil leaks in car engines is Lucas Engine Oil Stop Leak. Reviews of this product note its effectiveness in handling both oil pan and rigid seal leaks.

As with most oil leak preventatives, this Lucas product works best on small leaks where there is not too much damage to the seal. It also requires time to work, so users must add it regularly to the oil supply, replenishing it at every oil change at a minimum. More extensive seal damage can cause oil leaks, and Bar's Concentrated Rear Main Seal Repair is designed to address these issues specifically.