If your car is not in the first couple of hundred miles (use the oil
that came with it to do initial break in), change to whatever your
want.  It's all compatible - question is why do you want to use
fossoil?  Synthetics are a much better choice (unless you are leasing
the car and are not going to keep it - then do what ever), why do you
think that posche, bmw, ferrari, and almost every real race car runs
em?  They are better - send some oil to spectrol (lab in ohio(?)) to
figure out how fast your bearings, guides etc are wearing - cheaper in
the long run and will give you less wear in your motor.  Regular oil is
cheap in the short run, but you are obviously interested in cars, and if
you run a mustang, you can make it last by doing proper maintainence -
I've regretted selling mustangs, never will again.

Andrew