I think one of the preserved USATC S160s (no.2253) has recently been converted to oil firing, so hopefully there'll be some appropriate exhaust.
Oil firing can be clean - the Swiss ex-Kriegslok 52.8055 was converted years ago by DLM in Winterthur and is said to be so efficient that it only requires one person to act as both driver and fireman, whilst the cab interior rarely if ever gets dirty.
Thing is, no other mainline standard gauge engine seems to have followed the DLM techniques for improved efficiency.