Depends on which seal and which model vehicle, but it involves replacing the bad seal. Rear main seals usually aren't that difficult - though again, depending on the model, it may involve dropping the transmission. Front seals or crankshaft seals involve pulling the harmonic balancer (if you have one), pulling the timing cover (if you have one) pulling the old seal from around the crankshaft and tapping the new one into place.
That would depend on what oil seal you are referring to. There are many oil seals.
replace the seal
There are several types of oil leak sealant for a 1992 Honda Accord. A few of the sealants that can be used are Bar Leak's seal repair, Lucas Oil, and Blue Devil oil leak sealant.
There is only one way. Find the leak and repair whatever is causing the leak. Bad gasket, seal, or broken part, whatever.
An oil leak on a 1995 Toyota 4Runner can often be repaired by replacing the damaged gasket or seal. Tightening the oil pan bolts can also fix some leaks.
The cost to repair an oil leak on a 99 Ford Escort will usually cost between $250 and $500 depending on the cause. A worn seal or gasket is much cheaper than a bent pan or a ruptured block.
A 2001 Ford escape differential leak is usually caused by a bad seal. Replace the differential seal. The new seal should stop the oil leak.
Cost of a crank seal leak repair
You probably need to drop the oil pan down, remove the front cover and replace the cover gasket and main seal.
The oil leak is probably comming from the top of the oil pan there is a rubber gasket that goes between the oil pan and the water pumb which would make it look like its comming from the harmonic balancer, try replacing the oil pan gasket this should fix the problem. New answer: There is a small round oil seal / Harmonic balancer seal that fits in the timing cover and the balancer slides through the center of it. That is probley were your oil leak is. Most of the time the balancer itself gets a groove wore in it over time from that rubber seal running on it. You can repair the balancer by buying a Harmonic balancer repair sleeve/ speedy seal. Also replace the front timing cover seal.
It depends on where the leak is and what is causing the leak.
If there is leaking around the base of the distributor, and you will need to replace the seal. The distributor oil seal is a paper gasket or rubber O-ring, and the distributor is removed to replace either one if the seal is leaking. It is easy to change, just remove distributor and change the seal.