About 3 weeks ago i replaced my rear main seal,everything has worked just fine since. No leaks or dripping of the oil.I went about 100 to 200 miles since i replaced it and i still need to change the oil. Well, this morning on my way to work the pressure droped to about 4psi according to my guage. but if i give it gas it goes up to about 8. And when driving it sits at 18 at 2000 rpm.its never done this before. Any ideas I plan on changing oil and filter tonight but what else can it be? Could it actually just be a bad filter of should i pick up a new oil pump?