The only time I would recommend going to premium is if you anticipate a high load situation that might induce knock (pre-ignition / pinging) if you ran regular. These include overloading the van with passengers & luggage, towing, extended hill climbing, particularly when combined with high outside temps and A/C use. Under these extreme conditions the knock sensor will retard the timing to protect the engine, but the downside is reduced power, reduced fuel economy, and an elevation in operating temperature. All bad.... Going to premium under these conditions could pay large dividends as the engine can run with advanced timing and squeeze the full measure from the fuel. But under normal load condition, my experiments match Dandaman's - little noticeable difference and money wasted.