Yes, you can fit 18s on AWD models like yours. Its on the RWD models that have larger brake rotors that require 19" wheels at the minimum. So if you decide to get 18s, you can either run all season tires like the Continental DWS which are fantastic in the snow or you can opt for dedicated snow tires (winter tires). with 18x8" wheels, recommended tire size would be 235 45 18 which is good considering you want a softer ride. You can also get a higher sidewall tire, like a 245 45 18 for an even softer sidewall.
Yes we have plenty of 17=18" wheel fitments for the 13' GS350 AWD F Sport. The most comfortable and resistant to potholes would be the 17" as this would give you the most tire side wall. Here is a link to all of the wheels we offer in 17" :
Zig Ziegler | The Tire Tire Rack
7101 Vorden Parkway
South Bend, IN 46628
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