Despite what others may argue, and I respect their opinion: I have seen where synthetic oil leaked where conventional oil did not. I filled my front differential with synthetic oil, Amsoil no less, and my pinion seal started weeping oil within a few days. A few unfortunate events later, and I had to remove the differential, I refilled it with conventional oil, the leak was gone.
I guess all of this is a moot point now, sorry to hear about your BII. I hope they have a special section in hell for people who steal and damage other people's cars.