most police garages demand oil changes at 3k. Yes the car is idled a lot, but it is brought up to temp almost daily and they tend to get very short OCIs. It probably saw conventional oil. And here we have evidence that even on short intervals conventional oils leave behind crud.
And yet how many threads to we have running where everyone is saying that synthetic will only make your wallet lighter? That there is no advantage to running it "in your application" which is similar to this one.
IMO hit it with 2 cycles of ARX, then switch to either an ARX maintenance dose in whatever oil you want, or go with a good true synthetic like amsoil or red-line.