Shell Askari is among the most crooked (notorious) fuel station of Karachi. You will hardly notice commercial vehicle getting fuel from it. For 2 wheelers their dispensers are segregated because that class know immediately if there is any game in qty or quality of fuel.
Water contamination in their fuel be it petrol / diesel is also very high.
Hascol ahead of TV station over Stadium road, & at old subzi mandi towards university, PSO at new m.a. jinnah road (at left after crossing jail flyover). are few i have tested. For diesel, petrol & hi-octane. their pouring value for money is 97% with very low contamination.
We have a Diesel Shezore truck in family so i know a bit about such outlets.