Hiya Noel,
I'm not a big one for faffing around with oil - in fact, as a mechanic, I'm probably the worst for changing oil / doing services regularly! 
(Cobblers kids and all that)
My wife and I rode 2x F650s (the carbed ones) from here to NZ (Iran / Pakistan were roasting hot 40+) and we had to use whatever oil we could get at the time.
The bikes were still running fine when we got to NZ, so shit oil / extended oil changes didn't appear to do them any harm (50,000 Miles total)
Maybe call BMW or Touratech as Waz suggested or make your own decision based on the websites below - I agree with Waz, I would have thought the 15w40 would be fine. 15w50 is readily available, so for peace of mind order some off the web or go to your local bike shop. 1 or 2 £30 oil changes has to be better than worrying about your engine all the time.
You may as well go for the 15w50 straight away - it's not going to make any difference (compared to 15w40) in Spain or Africa (or the UK) at this time of year.
The Castrol oil you bought sounds like car oil - Motorcycle oil is different to car oil - The clutch on a bike is wet (ie it runs in oil) so you'll probably find the car oil has additives that will make your clutch slip.
You need to make sure you buy oil designed specifically for bikes.
Halfords is generally not the place to buy stuff for your bike.
Have you looked at the F650 web site? there's loads of info on there about different oils etc for your bike.
http://faq.f650.com/FAQs/OILFAQ.htm#F650 Oil Choice
While you're there, check out the info on Water Pump seals - That may come in handy... 
http://faq.f650.com/FAQs/Water_Pump_Seal_Repair_FAQ.htm
Maybe you should go for something like Shell Advance Ultra 4 oil - I use it in our air cooled Ducatis. Check out the descriptions here - it may give you a feel for the different types available:
http://www.shell.co.uk/home/content/gbr/products_services/on_the_road/oils_lubricants/advance/product_range/four_stroke.html#shell_advance_ultra_4_1
As for fork springs... depends on how much dosh you have - Again, I generally just "make do" with what I've got.
There's loads of people out there that have to have the latest and greatest, but at the end of the day can't ride to save themselves (All the gear - No idea)
It depends on how much weight you'll be carrying - we had no problem with the std forks / springs on our F650s and we rode some very rough roads - We carried very little compared to some people though (2 panniers each and an Ortlieb bag for our clothes)
If you have the Touratech tanks for the front of the Dakar, then you may want to beef the springs up a bit.
If you're a demon off road rider, then you'll probably notice the extra performance between the std springs and the progressive ones. If you're not a demon, then IMHO you may as well keep the cash and splash out on some well earned beers after some hard days riding in Africa! 
If you're going to be carrying panniers etc on the back, I'd be more worried about the rear shock (we have a friend who has just travelled from here to Aus and blew his std Dakar shock somewhere in "the Stans" and had to get a replacement.
Rich from Garage Night used a Dakar to go from UK to Singapore - he replaced his std shock with a WP unit before he left and didn't have any problems.
I actually used his old Dakar shock in my F650 as it was longer / stronger than the original carbed F650 one. I didn't have any problems either.
My wife's std F650 rear spring broke in half on her bike, and we had to have a new one made in India to replace it (Only cost a tenner to have a spring made though - you gotta love India!)
Can talk more about suspension etc at the HUMM if you like.
So, in conclusion:
Have a read up about the different oils available and buy a 15w50 that you think is suitable (or recommended by the F650 web site)
Shelve the front springs idea (unless you have the Touratech tanks) and buy us some beers at the HUMM
Think about a replacement rear shock (Depending on how much mileage you'll be doing / how much luggage you have)
Replace your water pump before you go...
Then go and do some riding!
Cheers
Trent
