I filled up at costco yesterday. Price was too good to pass, 3.49 for 92gas with extra 3% discount. I was a little worried about the gas quality so I did a little data logging. 10 mins of driving including multiple WOTs up to 6300RPM. Pulled the log to my computer and wow. I see 3.5 degree of ST KNK! I have been keeping track of different gas brands. So far my results are:
Chevron: 0 ST retard. 14 fill ups
Shell: 2~2.5 ST retard. 5 fill ups
76: 0 ST retard. 3 fill ups (This 76 station is right next the costco I filled up yesterday)
Costco: 3.5 ST. 1 fill up.
All the gas are 92 grade. At least from my data sets, Costco gas surely is worse than other named brands.