I've run 2.5 tanks with Millers eco-max in 1.5x the dose (the label says it can be used up to 2x the dose if you want something "extra"). The OBD doesn't indicate any improvement on this third tank on regular journeys - the same ~30-33 going to work (uphill) and the same ~35-40 going home (downhill) depending on traffic which is usually cr@p.
Once the millers runs out (one more tank maybe) I have some Redex 0-60 Cetane booster to try - a couple of bottles I got on a bonus card, seemed rude not to as they were free
