1993 BMW 325is OIL CHANGE FROM SYNTHETIC TO REGULAR?

I went back to the shop and requested them to change the oil back to synthetic but they said no they wouldn’t do it and that I shouldn’t have put synthetic oil in the motor to begin with and that it causes problems and they also said that it looked like I would need a new head gasket within six months but they did not tell me this when I first brought the car in for them to do the valve cover gasket that was leaking a small amount of oil (the oil change was done when they replaced the valve cover gasket)
also I was wondering. Why would they do the valve cover gasket if they noticed the head gasket needing to be replaced. And I was wondering if it was even possible to predict a blown head gasket before it actually happens.
The car ran great before I took it to them for a sudden starting problem that they also say was a faulty battery (and I know the battery was fine since it was an interstate red top sealed battery that wasn’t very old)
the whole thing is a rather long story so ill cut it short and get to my main question witch is. Is there anything they could of done wrong to the car to cause the head gasket to blow? And was it right for them to go through with the valve cover gasket if they knew the head gasket was about to blow and they would pretty much have to rebuild the motor anyway.
I think they took advantage of the situation and it just doesn’t seem like everything was done honestly.

Any opinions or advice would be appreciated

i can email a more detailed description of the whole story which is way to much to list here if neccessary