5-40 or 10-40 doesn't realy matter the probelm is 10-40 "only" protects down to -20 degrees so if outside temsp will be lower the oil ins't protecting as its to thick.
I'd have no worries about using either. Renault say 5-40 as it will in theoery be better on the engine when its cold and best if the engine being used for very short journeys when the cars not getting utpot temp but I woulnd't ever worry 10-540 is fine for the UK climite.
Theres a post here with the stuff you might want to read into
http://www.cliosport.net/forum/showthread.php?t=166251
I'd use Millers 5-40 as its great stuff takes a load of buse and I realy drive cars hard but for moet people elf pro silkine is more than enough cnhage it ever 6k.
The text you'll want from it is the text below which is the oil spec.
If anyone want the full PDF from there and cannot get it as there not a member on here PM me your emaill address and I'll send it. Its supposed to be for members only.
ACEA A2- 96/ A3- 96 15W40 or 15W50.
Oils of 10- 40 and even as low as 0- 30 can
be used. 10- 40 is the most commly used
oil.
ACEA A1- 98 is the standard oil.
ACEA A1 is the oil for fuel economy in
10W10, 10W40 or 10W- 50.
For none EC companies API SH/ SJ 15W-
40, 15W- 50 is the spec you want. API SJ-
IL SAC GF2 standard