I use acetone and 2 cycle ashless oil, mixed. I tried PS and noticed a drop in mileage. I keep PS handy for temps below 20f only.
My overall mpg average for the 05 Passat was 34.5 with PS. Without any additive for 10k miles, my overall mpg went to 36.2 mpg. With the acetone and 2 cycle mix, my last 10k average has jumped to 38.6 mpg. All these readings are based on the dash readout, and I do vent .
IMHO, PS, like tire chains, are nice insurance when the weather calls for it, otherwise good maintance will give you the greatest return on your investment in long troublefree miles. I use acetione for a more complete burn and less cylinder wash, and 2 cycle ashless oil for its lube properties of the injection system.