What my dealer told me about X5 oil!

[imalabil]

Hi all, newbie to the X5 world with the purchase in early October of a 2002 X5 3.0. I love the SUV, but it does leak and perhaps burn oil and I've had to add some in about 2K miles of driving (84K when purchaed from a BMW dealer). The dealer changed the oil prior to my purchase, and when I called to ask them what oil they used, they told me..

Full Synthetic, 10W-30, as recommended by BMW.

10w-30 in Indiana? That seems too thick to me. Does BMW even brand a Castrol in this viscosity? I asked him if he was sure, but I think he's wrong!
I've added Syntec 5W-30 and M1 5W-30 to top off, which I don't think will be a problem. I guess I'll find out.
In the meantime I've added some oil dye and loaned out a black light from Autozone to see where my leak is coming from. Vavle cover gasket sure would be easiest to fix...
BTW, I don't see any history in the Carfax of changing the OS. Couldn't you tell if it's bad by taking off the oil cap and seeing if there is a huge amount of pressure escaping?
Bill
BTW, great, informative forum. Because of you all I'm going to do the VC gasket change myself (if indicated), will check my CV joint boots regularly, will monitor for OS failures, etc.