v8 thor engine oil

I appreciate this is probably a boring one, but what oil can I use in my 2001 v8 thor engine? there are so many options, but I just want to change the oil to just make the engine just a little more quiet, so should I looking at mineral based, semi synthetic? I don't know what was already in the engine but after an oil change when I replaced the oil with mobil 10/40 the engine sounded different like constant ticking / tapping sound, not bad but noticeable but never heard this before the oil change. thoughts on this please?

We use a 10W/40 semi-synthetic diesel oil in ours, have done since we fitted a short engine in 2009.

Clears out any remaining crud in the engine over a period of time and keeps the insides clean.

The one we use is Chevron Supreme and we buy it in bulk at Costco.

Peter