It's a stretch, but are you running a PCV, and do you have a breather on it? My running oil pressure improved significantly when I added a breather cap on one cover with the PCV on the other side (had only the PCV before, sucking vacuum on the crank case which was bad). I assume it's a flat tappet cam, right? If you can run it with the covers removed can you press down on the lifter side of the rockers one at a time and see if your "tick" noise changes? Any visible difference in the rocker arm movement from cylinder to cylinder? Wondering about a flattening cam lobe.