Check the o-rings at the tip of the oil filter housing shaft. There are two of them. If they are hardened, or no longer round, they should be replaced. Proper o-rings can be found in any metric o-ring set.

It is good practice to replace these every third or fourth oil change as preventative maintenance.

The ones shown here are very flat, and brittle. They had to be broken off as all of their elasticity was gone. They had been neglected for some time, and they were doing little good as a seal.