Oil travels a lot when you're driving. The fan and wind from driving will push it all around the engine bay.
The valve cover gaskets are the #1 cause of oil leaks on these beasties. It's very possible that you have a valve cover gasket leak that is hiding under the intake manifold. The leaky bolt you mentioned is also a likely culprit. It may not seem like much, but while you're driving it will be blown around and as a result yo may have a larger leak than you thought. You could try to torque the screw down to specs and see if that helps. You could also order a valve cover gasket set (they're pretty cheap) and install the passenger side. This will let you know the condition of the gaskets. They seem to last about 5 to 10 years. It's a very common maintenance repair on the gen1 Xterra and Frontier. I also recommend replacing the grommets that go under the screws when the gaskets are replaced. The rubber grommet helps to maintain constant pressure on the gasket, but as the rubber ages it becomes hard and no longer functions as it should.
I also had an oil leak around one of the exhaust manifold studs on the drivers side. Don't ask me how, but my guess was that the stud enters one of the oil galleys in the block. There was a slow drip constantly on the end of the stud. I pulled the stud, put a high temp thread sealant on it, tightened the stud and nut and the leak went away,
Also check the back of the heads. There are two covers back there for the cam shafts. If the gaskets are bad or the bolts have come loose it will leak oil (mine did). It's an extremely tight squeeze between the engine and the firewall. I used a small mirror to inspect and that's how I found it.
Check the bottom of the timing cover. There's a little weep hole there. If there is oil dripping there you should replace the timing belt, cam and crank seals, and usually the water pump. A leak in the front seals will allow oil to leak out the weep hole and this oil can be blown back to the exhaust.
You could also put UV dye in the oil and after driving use a UV light to make it fluoresce. Make sure that it is intended for motor oil as AC or coolant dye should not be put in the oil. Note that some motor oil does fluoresce slightly under UV, but the dye will be very bright in comparison making it easy to discern the newly leaked oil from old.
As mentioned, the power steering hoses to and from the reservoir are known to leak as they age. If there is any oily residue at the ends of those hoses, then it is time to look at swapping them out. Basically the inside rubber breaks down and allows the PS fluid (ATF actually) to reach the inner fabric weave and wick down to the ends of the hose where it drips.
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