Oil pressure guage question...

[Thor06]

Hey guys, I feel like a fuckin moron asking this question, but I feel I need to ask it. Since getting the car running, I decided I need to be able to keep a decent eye on my engine so I go a boost, water temp, and oil pressure guage. I got the boost in just fine and the water temp wont be hard to get in, but I am having a few issues with the oil pressure guage. I know where the oil pressure sender is on the filter housing, but the fitting deal I got is just a tick too big to get into the stock oil pressure sender units hole. I figured the fitting from the pressure guage was a standard size where the stock oil pressure sender unit's hole was metric, am I right? Also, is the oil pressure sender's line supposed to fill up with oil? Does it just do it automatically or is there something I gotta do? Thanks guys.