quality of gas ... or lack of it !

[Q.R.]

Ok usually I fill up at Sunoco with 94 both cars (A4 1.8T and 90 v6 12v) 
last 2 times for the 90 I filled at Shell with 91 (V-power or lack of 
it) those 2 times I noticed that the car did not run as nicely as on the 
94 but it was OK. (Shell is much closer) Since my 90 is rated for 
minimum 91 oct, it should be OK.


Today I filled up at the same Shell station too but right after I left 
the station I noticed that my engine was running a lot more rough, I can 
feel the mis-fires !!!


The station is on a hight traffic road so it's not as if they have stale 
gas...


Could it be that they are selling 87 or 89 as 91 ?