If the leak is a static one and car has stood for a long time without the engine running, on some engines, the oil tends to drain down from the tank into the sump. If this is the case the oil level in the tank will be low. If you start the engine with a lot of oil in the sump it can cause a big "burp" of oil out of the air filter through the breather, better to drain the sump and put the oil back in the tank first. Once the sump is "dry" again the seal shouldn't leak statically. Always check the oil level in the tank after the engine has run for a few minutes, to scavenge the oil back from the sump.
If it is leaking when the engine is running it will need the crank seal replacing. Have a check of the cam ones to see if they are OK if that job is required. There was a bad batch of crank seals some time ago.
