well, switch to RON 92 , it isnt very expensive now. use good quality lubes, liqui molis and mobils. read the manual, read the book.
i think CAT is not fuel related, there must be some sensor calib, check issue
i owned a swift 1.3 DLX for 4 + years, i faced knocking issue once last year ( odo at 40,000)where the engine started to cough at <1000 rpm . turned out my throttle body needed cleaning and the local fuel station had messed up on his fuel checks . on crossing 50,000 km, the engine( swift) started making a weird clink clink audio note . im considering selling 
im not advertising the vitz here, but i believe the " knock " issue is being blown out of proportion , ive seen kids here at pakwheels put the 20w50 lube and then complain heavy feel and low mileage. just saying , but i just find the vitz being much better built, much better quality wise and much better to drive
price wise, well, you dont get standard ABS and airbags and EBD on the swift too, and no CVT as well !