Regarding your question about a lifter causing a drop in oil pressure, the answer is yes. Metal from the lifter and the cam can damage your bearings.
Drop in oil pressure is not a good sign but before jumping to any conclusions, need to rule out a rod bearing problem. While engine is cool, loosen plug wires. Crank engine then one at a time, pull plug wire away from plug (start with number one). Rev engine slighty and listen. Replace wire and move to the next. If knock gets quieter, not good.
Drop in oil pressure usually indicates a main bearing problem. Remove oil filter and pour out contents. If there is a problem, will notice small copper flakes in the oil. If suspicious, drop the oil pan to find out for sure.
Hopefully, something else is causing your problems. Truely wish you the best of luck.
Richard
