not sure of quicksilver's reasons but here are a few of why to avoid them:

-do not wrap your seat so it won't get dirty
-they don't really wash their hands...and they touch everywhere in your car
-although the service is fast...there are reasons why (quality)
-may use generic oil
-may steal from you (happens more frequently than you may think)

more technical things here http://www.jiffylubeproblems.com/
although i'm not sure how reliable it is...

...it's always safer to go to the dealer

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