What should I ask

If a vendor tells you that adding the ECO Fuel System will void a Warranty tell them to put it
in writing.

If they put it in writing please let us know, they are violating Federal Law (Magnuson-Moss Warranty Act). 

Many Vendors, suppliers and dealerships earn a large sum of money by selling replacement parts. The
ECO Fuel System in no way 
modifies or alters any motor. It through a patented process increases the Vapor
Pressure (Reid Vapor Pressure) in fuel helping it to burn more efficiently.  See MIT Technology Review 

99%+ of the time a dealer,vendor or motor manufacturer will not put it in writing that it will void a warranty. If you
install any aftermarket product it is advised that you check out their history, reputation and insurance. 
In the
event an aftermarket product causes damage they are responsible for the repair, not the manufacturer. 

The ECO Fuel System has been treating Internal Combustion motors for +/-15 years, see who uses it.

Example of a well know manufacturer with problems, FRAM Filters. 
1. Engine Filter Study (click here)

The ECO Fuel System has been installed on tens of thousands of motors (diesel, gasoline, natural gas, propane,
etc.) of all manufacturers.
The ECO is insured and has had no claims or issues since its inception in 1999, BBB A+ Rating.