ticking and oil pressure

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I have a 91 with 178000 miles. For a long time it has been ticking right after cold start up but stops after a few seconds. A coupe of days ago the ticking got much louder and I noticed the oil pressure gauge was all the way down. I pulled over checked the oil it was full re started it and and the gauge went up, then back down..... but for the most part it is down. What could this be? How long do oil pumps last? How expensive are they to change? How do I hook up a real pressure gauge? Has anyone had their oil system flushed? how does this work? This explorer is in good condition with many new parts and is for sale in the seattle area if any one would possibly be interested.

 

[D]

To add to the post above, a few days before it started ticking loudly and the pressure gauge started moving from nothing to normal erradically I changed the oil with Havoline 5-30 and added a bottle of cd2 oil detergent to see if that would fix the ticking it did have. Has any body used the cd2 is it bad? Also I just noticed when the engine is idiling it registers oil pressure then when you rev it at all it drops off untill it idles again for a few seconds. Also there is oil that has been accumulating on the bottom of the engine, so if the rear main seal was bad could that cause the oil pressure to fuctuate? Any one know if thicker oil or additives could stop it from ticking temporarly, I am planing to trade it in at a delear for a very nice 2000 ranger on weld rims with 32s.