notarealjeep

I don't know how to start this question so I'm just going to give you the run down of whats going on. I currently have a 1999 Jeep grand Cherokee it has 130,000 miles its one of the first GC made in 99 it was manufactured January 2nd 1999, i purchased the car with 94,000 miles on it driving like a champ never had a single problem had the oil changed every 3k and tires and rotors changed when they needed to, any who recently iv been having a problem with the oil pressure, i think... the gauge will read almost to the bottom line after starting it up after driving for driving for a while as soon as i start driving it climbs up to about 40 or so and sits there during the commute but when i start slowing down or stop it drops back down to really scary low. but it only does this maybe 1 out of every 3 or 4 times i drive other days it starts up and goes right to where it should be and doesn't move unless I'm giving it the gas in witch it will increase a little but normal. over the last 10k miles or so iv been getting the normal oil change but having them put a Lucas additive because this is really good for engine maintenance especially with older cars with quite a few miles, like mine... this symptom has been going on for a few months and the car seems to be preforming like the day i picked it up so i didn't think much of it because it never got to zero and car preformed the same even when it was low or when the gauge showed nothing was wrong but in the last few weeks theirs been this slight knocking noise you can barely hear it if you pop the hood and get really close to the hood you can hear it but very faintly. My friend who i like to think knows quite a bit about cars say i need a new motor he says i have about another 1,000 miles before i blow a rod or a cylinder. but the reason i bring up the oil pressure is because the noise goes away once the oil pressure goes up and the car warms up it seems the only time i can hear it is if the car is parked and i have my head against the hood... i haven't noticed and oil leaking i don't have white exhausts and the car seems to be preforming the way it should. I don't want to come off as one of the those people that are the wanting to hear there's nothing wrong (witch would be great if there isn't lol) but i was just hoping with you guys/girls being around these cars and there issue more than me or my friend you knew that it could be or if it is the engine needing to be replaced and if theirs a way to make sure before i drop like $1800 on a new engine and some one to install it. Thanks for any responses in advance!