not knowing what car you have its hard to say but--------
i think you are changing it too often. you can safely get
10,000 miles on yer car if you are using mobil 1 15/50 or 0/40.
i would recommend otherwise twice a year. if you don't use the
car much then once a year is fine.
you are using the fleece filters if it applies, right?
i'm also assuming that the rest of the car is maintained.

george