If you aren't seeing good pressure, there is either a blockage or the oil pump has failed, or even something exotic like the pickup falling off. In any of those cases, the oil pan will have to come off for a look.
Why was the oil pump replaced?
when i had this car we had to change the oil pump because the pump relief valve was stuck open. i've checked it with a mechanic before taking anything off.
mileage: it has 130000 on it. the past owner said he replaced the pan only at one time.
i hope i'm not really into a big trouble having purchased a used car such as this.