The first thing you should do is get the codes read and then post them here. Is the SEL flashing or solid?

Seems like cold weather startup, oil seperator clogged,oil past the valve cover gaskets and coils soaked with oil.

Just came back from the dealer, who agrees with most of you: the low rpm rumble is caused by ignition coil missfire in cylinder #6. They swapped #6 and #1 to double check and got the same fault code for #1. Good enough for me. Now I just have to figure out which one is #1.
BTW, my SES light is solid except that it blinked for a few seconds yesterday when I punched it to catch a yellow. And no, definitely not a cold weather issue in Fort Lauderdale.
So, any suggestions on best place to purchase a replacement ignition coil? What about recommended brand?
Thanks for all the help.

The fault sounds like Intermediate Lever problems in the valvegear of the N62 V8 engine. Do a search on "Intermediate Lever Issues". The ignition coil "failure" is a false trail and may not be actually happening if you read all the posts in the thread. Unfortunately, it is a big & expensive (unless warranty covers it) job to replace the 16 levers. It is usually only apparent in start-ups in very cold weather.

His is an 03, so pre-valvetronic... sounds like a coil to me. If you're just gonna replace one coil you could just get one from the dealer, should be in stock. And cyl.1 is the front cylinder on the passengers side.

Good news. Replaced the ignition coil on cylinder 1 and X5 back to running normal. Thanks for all the help.