Looking at the body contour around the front wheel well, it appears that they are designed to create a negative pressure and suck air out of the engine compartment and around the body.
Certainly use synthetic oil in the engine.
Don't use synthetic oil in the transmission! It has been tried and in the case of the Insight produces difficult shifting. The effect is like having a non synchromesh transmission.
Use only Honda MTF as it is specifically formulated for this task.
The wheel covers help or they would not have been worth the weight penalty. Quantifying the amount would be a job for a wind tunnel. The underbody pannels are even more important and are used on the Prius as well. Honda suggests that weight reduction, friction reduction and aerodynamic factors account for a third of the Insights doubling of gas mileage over a similar sized car. My guess would be that the total aerodynamics package acounts for 5 MPG.
The biggest factors for hypermilling are patience, practice, persistance, concentration and experience. Combine these with weather, speed, road condition, temperature, traffic, and especially the length of your trip. Sadly these cannot be bolted on or poured in.
Fortunately they work on any vehicle.