What wattage oil pan heater do you guys recommend me?

Start with the BMW corporate offices in Canada, then move up the chain to the real factory for the M-cars in Europe if you don’t get satisfaction at the Canadian offices. You really need to talk to someone technical, and I suspect that the real factory is where you will end up if you can get a number. You just might get it if you ask questions and make it clear that you want information and not a guarantee.

You could also have a conversation with @Docnick. He lives near Calgary IIRC, and worked in the oilfield equipment business. If anyone is likely to know a thing or two about your issue, he would. Reread his response above.