knock/slap, and oil pressure

so ive gotten to the point of driving my 91 cherokee 4L. ive changed the oil pump and pressure sensor. changed the oil twice, good n clean. at idle i get what sounds like a knock, but ive read on here about piston slap so i suppose it could be that, but my oil pressure doesnt hold at 40 when warm like what ive read from others. at idle warm its at 40 till i get on the highway n get it good n hot then it goes down to the line midway between 0 and 40. when i stop at a light after that it goes down below that line, not much below but still. it doesnt knock under load not that i can hear anyway. what do you guys think? its not hard to change main bearings n rod bearings. im driving this thing from alaska to arizona so i wanna make sure its good to go before i get to canada...yikes. when i pulled the pan to change the oil pump i got a good look at the cam and it didnt seem worn out of the ordinary...where is the cam in the oil food chain? im using 5-30 like the book says...if i put 10-40 in n it went away would that be a good way to tell if its piston slap or not?

First thing I would do before planning any repairs is to verify exactly what your oil pressure is by installing a mechanical oil pressure gauge where the sending unit is. You could easily have a sender problem as they are very common on the 4.0

Testing only takes a few minutes and you can rent this test equipment from some of the big box stores for only a few bucks. If pressure is good with the mechanical, then install a new sending unit (aka: oil pressure switch in some parts stores). At hot idle, your 4.0 should not be much less than 13 psi

Higher mileage noises include a bit of piston slap or lifter noise, neither by themselves are particularily dangerous but of course you would need to keep an ear on them if they were to worsen.

That line would mark 20 psi. My '88 runs around 18psi on a hot idle. Books says 13 is the low limit.

If you're not hearing any loud knocking beyond what any high mileage engine would produce, I'd say you're still good.

thanx guys i really appreciate your input. as my username implies ive been a gm guy all my life, n things are different in the jeep world. alot of it i like, i couldnt believe i changed my oil pump with a jack and a mostly missing socket set, and did it in less than an hour and a half! everythings real easy to get to, well compared to my old truck. but theres noises n shifting differences that really had me worried i'd bought someones abused redheaded step car. seems everything ive been fretting over is normal though, im a happy camper.