Inside of the oil filter shaft that is attatched to the shaft there may be a 1 way check valve. If you are going to blow it with an air gun blow into the small hole that is up near the top (filter cap).
See if your can find your engine on this site and take a look at the oil system.
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Main.html
When you changed your oil did you also change the Wt (viscosity)?
Do you have a mechanical gauge or and electric one?