2000 Ford Explorer - Low Oil Reading then normal.

: I have 114k miles and have done oil changes at 5k intervals using Mobil 1 synthetic oil 10w-30. Never run the engine hard or in extreme conditions, just the normal everday commute and travel.

About a month ago my my Explorer's oil pressure gauge read low w/the check gage light on at start up in the morning and then pop up to normal levels sitting below the H and L makers after a few minutes.

Remains there during the rest of the driving time and during other start ups in the day. I use purolator and fram oil filters only.

I live in West Covina, CA and the moring temps have been in low 40's during the night and early morning. This past weekend I did an oil change going to a lighter oil 5w-30 mobil1 synthetic. SInce then I had to start ups on read low before popping to normal and this morning it popped up to normal immediately.

I did not notice any leaks around the oil filter, but just behind the oil pan on the trans pan I had some oil/grease residue, but I clean that off completely to see if it reappeared indicating to me a possilble leak somewhere.

For FYI -where can I find the oil pressure sensor and replace if necessary and where is the oil pump if that is failing and needs replacement? And how hard are they to replace? Any help would be greatly appreaciated.