Hi again,
Wow! That is a LOW mileage car! No wonder you are concerned with the knock! I have a suggestion for you. It's possible that the Mobil one doesn't have the viscosity the engine is used to. You are in a fairly warm area for most of the year, so I recommend that you change the oil and filter, putting in 20-50W. I'm thinking that probably it didn't knock until the oil was changed to the Mobil One, and the new oil is thin enough so it allowed the noise to surface. This idea won't cost you much, and might take care of the noise. I have been using 20-50W Valvoline in all my vehicles for many years, and one of them has over 350K with no problems! (The 280Z, wouldn't you know it's the Japanese car that has held up!) (I live near Portland, OR, where the climate is moderate all year long, though - I wouldn't recommend 20-50W in a real cold climate in the wintertime.) As you probably know, your engine has a single row timing chain, and they went to a double row with the later engines, after they discovered the single row had wear problems that showed up around 100K miles. It is MUCH cheaper to have the chain and sprockets replaced while the engine is in good shape than it is after the chain/sprocket has slipped, allowing valves and pistons to collide. So you should think about having that job done somewhere around 80K or so....
Regards, Richard Wooldridge