Mr. Tire Potentially Ruined My Car Moon TWP PA

I've been taking my car here for a while for oil changes, until recent...
I went in for your everyday oil change; The sales guy John I believe his name is, talked me into (basically forced) me to change to semi synthetic, even after stating that I did NOT want to change.
The day after I noticed I was leaking oil pretty significantly, since they were closing I had to wait until they opened the next day, making me late to work (which is super neat) They put a new gasket and plug in saying that mine was cracked and that it's all fixed.
JUST KIDDING!!
Two days later I noticed I was leaking oil even worse than before; called them; left work early (again super neat) only for them to tell me now that I have a rear main seal break which is roughly anywhere from $1200 to $1500 repair.. 
Hmm, how does something like that happen to an car thats only 6 years old?
and
How did you not catch this the first time?
or how about
That the oil they showed me that was leaking was black, and clear as day the oil leaking from my car is unused oil...
So I e-mailed corporate, their answer? I wouldn't know I've been waiting over a week to hear back!!!