1995 Nissan Pathfinder lifters and engine sludge

So I took my car to the mechanic about a small clicking sound when it accelerates. I also needed an oil change. My mechanic called me back telling me that I had a serious engine problem. Apparently when he drained the oil and changed it, the sound got worse. He concluded that the lifters were bad. He did some tests to see if it was the oil pump with an oil pressure problem, but he said it was fine. His end conclusion is that the lifters are damaged due to lack of oil change. I asked him if I could get some engine cleaner that gets rid of sludge. He said he had used some engine cleaner, and showed me the bottle. He also said the reason why the sound was not louder until after toe oil change was because there was lucas oil additive. So he quoted me $600 to replace the lifters, and put some more lucas in it in the mean time.

Q: if I wanted to put some of my own engine cleaner specifically for lifters and sludge. One can I do that with the lucas in it? And two can you tell me where the crankcase is. The bottle says to pure into the crankcase.