15w40 is getting pretty thick for below -10C. That said, just give it some time to run and warm up, please don't do what I have seen some people doing and rev it right up after starting -- that is not good for any engine, but worse for diesels. By the time you get to your next oil change it may be getting into the hotter weather. I run Shell Rotella T6 full synthetic 5w40 year round, so it is really up to you.
With good glow plugs/batteries and fuel with cold weather additive, you should not have any issue starting up. I have started my diesels in weather colder than -30C with just a couple of glows and fire it up. You 'might' have to do that more than once if it stalls out, but that is pretty rare.
I am all about preparing for every potential possibility so if you do have a block heater in your cruiser, and you have a little 15amp or higher generator, you could always take the genny ''just in case''...
In an absolute emergency situation, make sure you prep a list of mud members near where you will be staying and make sure they are on speed dial!
Seriously though, if you follow the advice/tips listed here you should have a pleasant and restful holiday enjoying the wonders of hot springs in winter!!! Havve fun!